diff --git a/app/assets/javascripts/discourse/controllers/create_account_controller.js b/app/assets/javascripts/discourse/controllers/create_account_controller.js
index 7f48f718f..81b1cb521 100644
--- a/app/assets/javascripts/discourse/controllers/create_account_controller.js
+++ b/app/assets/javascripts/discourse/controllers/create_account_controller.js
@@ -15,6 +15,7 @@ Discourse.CreateAccountController = Discourse.Controller.extend(Discourse.ModalF
   accountChallenge: 0,
   formSubmitted: false,
   rejectedEmails: Em.A([]),
+  prefilledUsername: null,
 
   submitDisabled: function() {
     if (this.get('formSubmitted')) return true;
@@ -95,6 +96,28 @@ Discourse.CreateAccountController = Discourse.Controller.extend(Discourse.ModalF
     });
   }.property('accountEmail', 'rejectedEmails.@each'),
 
+  prefillUsername: function() {
+    if (this.get('prefilledUsername')) {
+      if (this.get('accountUsername') === this.get('prefilledUsername')) {
+        this.set('accountUsername', '');
+      }
+      this.set('prefilledUsername', null);
+    }
+    if (this.get('emailValidation.ok') && this.blank('accountUsername')) {
+      this.fetchExistingUsername();
+    }
+  }.observes('emailValidation', 'accountEmail'),
+
+  fetchExistingUsername: Discourse.debounce(function() {
+    var self = this;
+    Discourse.User.checkUsername(null, this.get('accountEmail')).then(function(result) {
+      if (result.suggestion && self.blank('accountUsername')) {
+        self.set('accountUsername', result.suggestion);
+        self.set('prefilledUsername', result.suggestion);
+      }
+    });
+  }, 500),
+
   usernameMatch: function() {
     if (this.usernameNeedsToBeValidatedWithEmail()) {
       if (this.get('emailValidation.failed')) {
@@ -119,6 +142,13 @@ Discourse.CreateAccountController = Discourse.Controller.extend(Discourse.ModalF
   basicUsernameValidation: function() {
     this.set('uniqueUsernameValidation', null);
 
+    if (this.get('accountUsername') === this.get('prefilledUsername')) {
+      return Discourse.InputValidation.create({
+        ok: true,
+        reason: I18n.t('user.username.prefilled')
+      });
+    }
+
     // If blank, fail without a reason
     if (this.blank('accountUsername')) {
       return Discourse.InputValidation.create({
diff --git a/app/controllers/users_controller.rb b/app/controllers/users_controller.rb
index 42beb0386..26c52b86d 100644
--- a/app/controllers/users_controller.rb
+++ b/app/controllers/users_controller.rb
@@ -97,7 +97,10 @@ class UsersController < ApplicationController
   # Used for checking availability of a username and will return suggestions
   # if the username is not available.
   def check_username
-    params.require(:username)
+    if !params[:username].present?
+      params.require(:username) if !params[:email].present?
+      return render(json: success_json) unless SiteSetting.call_discourse_hub?
+    end
     username = params[:username]
 
     target_user = user_from_params_or_current_user
@@ -107,7 +110,7 @@ class UsersController < ApplicationController
 
     checker = UsernameCheckerService.new
     email = params[:email] || target_user.try(:email)
-    render(json: checker.check_username(username, email))
+    render json: checker.check_username(username, email)
   rescue RestClient::Forbidden
     render json: {errors: [I18n.t("discourse_hub.access_token_problem")]}
   end
diff --git a/app/services/username_checker_service.rb b/app/services/username_checker_service.rb
index 24ec615e9..71c6d9a2b 100644
--- a/app/services/username_checker_service.rb
+++ b/app/services/username_checker_service.rb
@@ -1,15 +1,18 @@
 class UsernameCheckerService
 
   def check_username(username, email)
-    validator = UsernameValidator.new(username)
-    if !validator.valid_format?
-      {errors: validator.errors}
-    elsif !SiteSetting.call_discourse_hub?
-      check_username_locally(username)
-    else
-      check_username_with_hub_server(username, email)
+    if username && username.length > 0
+      validator = UsernameValidator.new(username)
+      if !validator.valid_format?
+        {errors: validator.errors}
+      elsif !SiteSetting.call_discourse_hub?
+        check_username_locally(username)
+      else
+        check_username_with_hub_server(username, email)
+      end
+    elsif email and SiteSetting.call_discourse_hub?
+      {suggestion: DiscourseHub.nickname_for_email(email)}
     end
-
   end
 
   # Contact the Discourse Hub server
diff --git a/config/locales/client.en.yml b/config/locales/client.en.yml
index c5e440317..0fe64bda2 100644
--- a/config/locales/client.en.yml
+++ b/config/locales/client.en.yml
@@ -280,6 +280,7 @@ en:
         too_long: "Your username is too long."
         checking: "Checking username availability..."
         enter_email: 'Username found. Enter matching email.'
+        prefilled: "Email matches this registered username."
 
       password_confirmation:
         title: "Password Again"
diff --git a/lib/discourse_hub.rb b/lib/discourse_hub.rb
index 9e699e184..e4e76f191 100644
--- a/lib/discourse_hub.rb
+++ b/lib/discourse_hub.rb
@@ -32,6 +32,11 @@ module DiscourseHub
     [json['match'], json['available'] || false, json['suggestion']]
   end
 
+  def self.nickname_for_email(email)
+    json = get('/users/nickname_match', {email: email})
+    json['suggestion']
+  end
+
   def self.register_nickname(nickname, email)
     json = post('/users', {nickname: nickname, email: email})
     if json.has_key?('success')
@@ -100,7 +105,7 @@ module DiscourseHub
     if Rails.env == 'production'
       'http://api.discourse.org/api'
     else
-      'http://local.hub:3000/api'
+      ENV['HUB_BASE_URL'] || 'http://local.hub:3000/api'
     end
   end
 
diff --git a/spec/controllers/users_controller_spec.rb b/spec/controllers/users_controller_spec.rb
index 41232e25c..4c66603b8 100644
--- a/spec/controllers/users_controller_spec.rb
+++ b/spec/controllers/users_controller_spec.rb
@@ -545,7 +545,7 @@ describe UsersController do
       DiscourseHub.stubs(:nickname_available?).returns([true, nil])
     end
 
-    it 'raises an error without a username parameter' do
+    it 'raises an error without any parameters' do
       lambda { xhr :get, :check_username }.should raise_error(ActionController::ParameterMissing)
     end
 
@@ -580,29 +580,19 @@ describe UsersController do
         DiscourseHub.expects(:nickname_match?).never
       end
 
-      context 'available everywhere' do
+      it 'returns nothing when given an email param but no username' do
+        xhr :get, :check_username, email: 'dood@example.com'
+        response.should be_success
+      end
+
+      context 'username is available' do
         before do
           xhr :get, :check_username, username: 'BruceWayne'
         end
         include_examples 'when username is available everywhere'
       end
 
-      context 'available locally but not globally' do
-        before do
-          xhr :get, :check_username, username: 'BruceWayne'
-        end
-        include_examples 'when username is available everywhere'
-      end
-
-      context 'unavailable locally but available globally' do
-        let!(:user) { Fabricate(:user) }
-        before do
-          xhr :get, :check_username, username: user.username
-        end
-        include_examples 'when username is unavailable locally'
-      end
-
-      context 'unavailable everywhere' do
+      context 'username is unavailable' do
         let!(:user) { Fabricate(:user) }
         before do
           xhr :get, :check_username, username: user.username
@@ -641,7 +631,7 @@ describe UsersController do
         end
         include_examples 'checking an invalid username'
 
-        it 'should return the "too short" message' do
+        it 'should return the "too long" message' do
           ::JSON.parse(response.body)['errors'].should include(I18n.t(:'user.username.long', max: User.username_length.end))
         end
       end
@@ -679,12 +669,20 @@ describe UsersController do
           include_examples 'check_username when nickname is available everywhere'
         end
 
-        context 'and email is given' do
+        context 'both username and email is given' do
           before do
             xhr :get, :check_username, username: 'BruceWayne', email: 'brucie@gmail.com'
           end
           include_examples 'check_username when nickname is available everywhere'
         end
+
+        context 'only email is given' do
+          it "should check for a matching username" do
+            UsernameCheckerService.any_instance.expects(:check_username).with(nil, 'brucie@gmail.com').returns({json: 'blah'})
+            xhr :get, :check_username, email: 'brucie@gmail.com'
+            response.should be_success
+          end
+        end
       end
 
       shared_examples 'when email is needed to check nickname match' do
diff --git a/spec/services/username_checker_service_spec.rb b/spec/services/username_checker_service_spec.rb
index 9d7375101..7f802ec6b 100644
--- a/spec/services/username_checker_service_spec.rb
+++ b/spec/services/username_checker_service_spec.rb
@@ -11,10 +11,6 @@ describe UsernameCheckerService do
     end
 
     context 'Username invalid' do
-      it 'rejects blank usernames' do
-        result = @service.check_username('',  @nil_email)
-        expect(result).to have_key(:errors)
-      end
       it 'rejects too short usernames' do
         result = @service.check_username('a', @nil_email)
         expect(result).to have_key(:errors)
@@ -40,47 +36,75 @@ describe UsernameCheckerService do
         SiteSetting.stubs(:call_discourse_hub?).returns(true)
       end
 
-      context 'and email is given' do
+      context 'username and email is given' do
         it 'username is available locally but not globally' do
-          DiscourseHub.stubs(:nickname_available?).returns([false, 'suggestion'])
-          DiscourseHub.stubs(:nickname_match?).returns([true, false, nil])
+          DiscourseHub.expects(:nickname_available?).never
+          DiscourseHub.expects(:nickname_match?).returns([false, false, 'porkchop'])
+          result = @service.check_username('vincent', @email)
+          expected = { available: false, global_match: false, suggestion: 'porkchop' }
+          expect(result).to eq(expected)
+        end
+
+        it 'username is available both locally and globally' do
+          DiscourseHub.expects(:nickname_available?).never
+          DiscourseHub.stubs(:nickname_match?).returns([true, true, nil])
           result = @service.check_username('vincent', @email)
           expected = { available: true, global_match: true }
           expect(result).to eq(expected)
         end
 
+        it 'username is available locally but not globally' do
+          DiscourseHub.stubs(:nickname_match?).returns([false, false, 'suggestion'])
+          result = @service.check_username('vincent', @email)
+          expected = { available: false, global_match: false, suggestion: 'suggestion' }
+          expect(result).to eq(expected)
+        end
+
+        it 'username is available globally but not locally' do
+          DiscourseHub.stubs(:nickname_match?).returns([false, true, nil])
+          User.stubs(:username_available?).returns(false)
+          UserNameSuggester.stubs(:suggest).returns('einar-j')
+          expected = { available: false, suggestion: 'einar-j' }
+          result = @service.check_username('vincent', @email)
+          expect(result).to eq(expected)
+        end
+
+        it 'username not available anywhere' do
+          DiscourseHub.stubs(:nickname_match?).returns([false, false, 'suggestion'])
+          expected = { available: false, suggestion: 'suggestion', global_match: false }
+          @nil_email = nil
+          result = @service.check_username('vincent', @email)
+          expect(result).to eq(expected)
+        end
       end
 
-      it 'username is available both locally and globally' do
-        DiscourseHub.stubs(:nickname_available?).returns([true, nil])
-        DiscourseHub.stubs(:nickname_match?).returns([false, true, nil])
-        result = @service.check_username('vincent', @email)
-        expected = { available: true, global_match: false }
-        expect(result).to eq(expected)
+      shared_examples "only email is given" do
+        it "should call the correct api" do
+          DiscourseHub.expects(:nickname_available?).never
+          DiscourseHub.expects(:nickname_match?).never
+          DiscourseHub.stubs(:nickname_for_email).returns(nil)
+          result
+        end
+
+        it 'no match on email' do
+          DiscourseHub.stubs(:nickname_for_email).returns(nil)
+          result.should == {suggestion: nil}
+        end
+
+        it 'match found for email' do
+          DiscourseHub.stubs(:nickname_for_email).returns('vincent')
+          result.should == {suggestion: 'vincent'}
+        end
       end
 
-      it 'username is available locally but not globally' do
-        DiscourseHub.stubs(:nickname_match?).returns([false, true, nil])
-        result = @service.check_username('vincent', @email)
-        expected = { available: true, global_match: false }
-        expect(result).to eq(expected)
+      context 'username is nil' do
+        subject(:result) { @service.check_username(nil, @email) }
+        include_examples "only email is given"
       end
 
-      it 'username is available globally but not locally' do
-        DiscourseHub.stubs(:nickname_match?).returns([false, true, nil])
-        User.stubs(:username_available?).returns(false)
-        UserNameSuggester.stubs(:suggest).returns('einar-j')
-        expected = { available: false, suggestion: 'einar-j' }
-        result = @service.check_username('vincent', @email)
-        expect(result).to eq(expected)
-      end
-
-      it 'username not available anywhere' do
-        DiscourseHub.stubs(:nickname_match?).returns([false, false, 'suggestion'])
-        expected = { available: false, suggestion: 'suggestion', global_match: false }
-        @nil_email = nil
-        result = @service.check_username('vincent', @email)
-        expect(result).to eq(expected)
+      context 'username is empty string' do
+        subject(:result) { @service.check_username('', @email) }
+        include_examples "only email is given"
       end
     end
 
diff --git a/test/javascripts/controllers/create_account_controller_test.js b/test/javascripts/controllers/create_account_controller_test.js
new file mode 100644
index 000000000..8e6ced64c
--- /dev/null
+++ b/test/javascripts/controllers/create_account_controller_test.js
@@ -0,0 +1,20 @@
+module("Discourse.CreateAccountController");
+
+test('basicUsernameValidation', function() {
+  var testInvalidUsername = function(username, expectedReason) {
+    var controller = testController(Discourse.CreateAccountController, null);
+    controller.set('accountUsername', username);
+    equal(controller.get('basicUsernameValidation.failed'), true, 'username should be invalid: ' + username);
+    equal(controller.get('basicUsernameValidation.reason'), expectedReason, 'username validation reason: ' + username + ', ' + expectedReason);
+  };
+
+  testInvalidUsername('', undefined);
+  testInvalidUsername('x', I18n.t('user.username.too_short'));
+  testInvalidUsername('1234567890123456', I18n.t('user.username.too_long'));
+
+  var controller = testController(Discourse.CreateAccountController, null);
+  controller.set('accountUsername',   'porkchops');
+  controller.set('prefilledUsername', 'porkchops');
+  equal(controller.get('basicUsernameValidation.ok'), true, 'Prefilled username is valid');
+  equal(controller.get('basicUsernameValidation.reason'), I18n.t('user.username.prefilled'), 'Prefilled username is valid');
+});