diff --git a/django/forms/forms.py b/django/forms/forms.py
index 9905eee7c7..97ee72e98e 100644
--- a/django/forms/forms.py
+++ b/django/forms/forms.py
@@ -297,9 +297,12 @@ class BaseForm(object):
 
     def _clean_form(self):
         try:
-            self.cleaned_data = self.clean()
+            cleaned_data = self.clean()
         except ValidationError as e:
             self._errors[NON_FIELD_ERRORS] = self.error_class(e.messages)
+        else:
+            if cleaned_data is not None:
+                self.cleaned_data = cleaned_data
 
     def _post_clean(self):
         """
@@ -315,7 +318,7 @@ class BaseForm(object):
         not be associated with a particular field; it will have a special-case
         association with the field named '__all__'.
         """
-        return self.cleaned_data
+        pass
 
     def has_changed(self):
         """
diff --git a/docs/ref/forms/validation.txt b/docs/ref/forms/validation.txt
index 87f9741c1b..c6d2722bf7 100644
--- a/docs/ref/forms/validation.txt
+++ b/docs/ref/forms/validation.txt
@@ -75,10 +75,8 @@ overridden:
   any validation that requires access to multiple fields from the form at
   once. This is where you might put in things to check that if field ``A``
   is supplied, field ``B`` must contain a valid email address and the
-  like. The data that this method returns is the final ``cleaned_data``
-  attribute for the form, so don't forget to return the full list of
-  cleaned data if you override this method (by default, ``Form.clean()``
-  just returns ``self.cleaned_data``).
+  like. This method can return a completely different dictionary if it wishes,
+  which will be used as the ``cleaned_data``.
 
   Note that any errors raised by your ``Form.clean()`` override will not
   be associated with any field in particular. They go into a special
@@ -403,9 +401,6 @@ example::
                     raise forms.ValidationError("Did not send for 'help' in "
                             "the subject despite CC'ing yourself.")
 
-            # Always return the full collection of cleaned data.
-            return cleaned_data
-
 In this code, if the validation error is raised, the form will display an
 error message at the top of the form (normally) describing the problem.
 
@@ -443,9 +438,6 @@ sample) looks like this::
                 del cleaned_data["cc_myself"]
                 del cleaned_data["subject"]
 
-            # Always return the full collection of cleaned data.
-            return cleaned_data
-
 As you can see, this approach requires a bit more effort, not withstanding the
 extra design effort to create a sensible form display. The details are worth
 noting, however. Firstly, earlier we mentioned that you might need to check if
diff --git a/docs/releases/1.7.txt b/docs/releases/1.7.txt
index fbbf93f627..6bf13834d2 100644
--- a/docs/releases/1.7.txt
+++ b/docs/releases/1.7.txt
@@ -127,6 +127,11 @@ Minor features
   for each individual field will be respected, and a new ``incomplete``
   validation error will be raised when any required fields are empty.
 
+* The :meth:`~django.forms.Form.clean` method on a form no longer needs to
+  return ``self.cleaned_data``. If it does return a changed dictionary then
+  that will still be used. The default implementation no longer returns
+  ``self.cleaned_data``.
+
 Backwards incompatible changes in 1.7
 =====================================
 
diff --git a/tests/forms_tests/tests/test_extra.py b/tests/forms_tests/tests/test_extra.py
index afdfaf5281..ba835495c6 100644
--- a/tests/forms_tests/tests/test_extra.py
+++ b/tests/forms_tests/tests/test_extra.py
@@ -620,6 +620,24 @@ class FormsExtraTestCase(TestCase, AssertFormErrorsMixin):
         self.assertTrue(f.is_valid())
         self.assertEqual(f.cleaned_data['username'], 'sirrobin')
 
+    def test_changing_cleaned_data_in_clean(self):
+        class UserForm(Form):
+            username = CharField(max_length=10)
+            password = CharField(widget=PasswordInput)
+
+            def clean(self):
+                data = self.cleaned_data
+
+                # Return a different dict. We have not changed self.cleaned_data.
+                return {
+                    'username': data['username'].lower(),
+                    'password': 'this_is_not_a_secret',
+                }
+
+        f = UserForm({'username': 'SirRobin', 'password': 'blue'})
+        self.assertTrue(f.is_valid())
+        self.assertEqual(f.cleaned_data['username'], 'sirrobin')
+
     def test_overriding_errorlist(self):
         @python_2_unicode_compatible
         class DivErrorList(ErrorList):