diff --git a/lib/mongo/connection.rb b/lib/mongo/connection.rb
index ece5c41..346fdcc 100644
--- a/lib/mongo/connection.rb
+++ b/lib/mongo/connection.rb
@@ -665,7 +665,7 @@ module Mongo
         user    = auth[0]
         pwd     = auth[1]
         db_name = auth[2]
-        self.db(db_name).authenticate(user, pwd) || raise(MongoDBError, "Failed to authenticate db #{db_name}.")
+        self.db(db_name).authenticate(user, pwd)
       end
     end
   end
diff --git a/lib/mongo/db.rb b/lib/mongo/db.rb
index cf0417c..b79a096 100644
--- a/lib/mongo/db.rb
+++ b/lib/mongo/db.rb
@@ -79,6 +79,8 @@ module Mongo
     #
     # @return [Boolean]
     #
+    # @raise [AuthenticationError]
+    #
     # @core authenticate authenticate-instance_method
     def authenticate(username, password)
       doc = command(:getnonce => 1)
@@ -90,7 +92,8 @@ module Mongo
       auth['user'] = username
       auth['nonce'] = nonce
       auth['key'] = Digest::MD5.hexdigest("#{nonce}#{username}#{hash_password(username, password)}")
-      ok?(command(auth))
+      ok?(command(auth)) ||
+        raise(MongoDBError::AuthenticationError, "Failed to authenticate user '#{username}' on db '#{self.name}'")
     end
 
     # Adds a user to this database for use with authentication. If the user already
diff --git a/lib/mongo/exceptions.rb b/lib/mongo/exceptions.rb
index 8d39148..a7990ce 100644
--- a/lib/mongo/exceptions.rb
+++ b/lib/mongo/exceptions.rb
@@ -46,6 +46,9 @@ module Mongo
   # when the document contains objects that can't be serialized as BSON.
   class InvalidDocument < MongoDBError; end
 
+  # Raised when authentication fails.
+  class AuthenticationError < MongoDBError; end
+
   # Raised when a database operation fails.
   class OperationFailure < MongoDBError; end
 
diff --git a/test/db_test.rb b/test/db_test.rb
index 3cbd9e6..9b08086 100644
--- a/test/db_test.rb
+++ b/test/db_test.rb
@@ -128,8 +128,12 @@ class DBTest < Test::Unit::TestCase
 
   def test_authenticate
     @@db.add_user('spongebob', 'squarepants')
-    assert !@@db.authenticate('nobody', 'nopassword')
-    assert !@@db.authenticate('spongebob' , 'squareliederhosen')
+    assert_raise Mongo::AuthenticationError do
+      assert !@@db.authenticate('nobody', 'nopassword')
+    end
+    assert_raise Mongo::AuthenticationError do
+      assert !@@db.authenticate('spongebob' , 'squareliederhosen')
+    end
     assert @@db.authenticate('spongebob', 'squarepants')
     @@db.logout
     @@db.remove_user('spongebob')
@@ -139,7 +143,7 @@ class DBTest < Test::Unit::TestCase
     @@db.add_user('spongebob', 'squarepants')
     assert Mongo::Connection.from_uri("mongodb://spongebob:squarepants@localhost/#{@@db.name}")
 
-    assert_raise MongoDBError do
+    assert_raise Mongo::AuthenticationError do
       Mongo::Connection.from_uri("mongodb://wrong:info@localhost/#{@@db.name}")
     end
   end
@@ -207,7 +211,9 @@ class DBTest < Test::Unit::TestCase
     assert @@db.authenticate("bob", "secret")
     @@db.logout
     assert @@db.remove_user("bob")
-    assert !@@db.authenticate("bob", "secret")
+    assert_raise Mongo::AuthenticationError do
+      @@db.authenticate("bob", "secret")
+    end
   end
 
   def test_remove_non_existant_user