diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 0404f2e315..1ed481bd45 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* [DOCS] Document :allow_nil option of #validate_uniqueness_of. Closes #3143 [Caio Chassot]
+
 * Bring the sybase adapter up to scratch for 1.2 release. [jsheets]
 
 * Rollback #new_record? and #id values for created records that rollback in an after_save callback. Closes #6910 [Ben Curren]
diff --git a/activerecord/lib/active_record/validations.rb b/activerecord/lib/active_record/validations.rb
index 342a4844cc..6f284242cd 100755
--- a/activerecord/lib/active_record/validations.rb
+++ b/activerecord/lib/active_record/validations.rb
@@ -520,6 +520,7 @@ module ActiveRecord
       # * <tt>message</tt> - Specifies a custom error message (default is: "has already been taken")
       # * <tt>scope</tt> - One or more columns by which to limit the scope of the uniquness constraint.
       # * <tt>case_sensitive</tt> - Looks for an exact match.  Ignored by non-text columns (true by default).
+      # * <tt>allow_nil</tt> - If set to true, skips this validation if the attribute is null (default is: false)
       # * <tt>if</tt> - Specifies a method, proc or string to call to determine if the validation should
       # occur (e.g. :if => :allow_validation, or :if => Proc.new { |user| user.signup_step > 2 }).  The
       # method, proc or string should return or evaluate to a true or false value.