diff --git a/phpBB/includes/acp/acp_disallow.php b/phpBB/includes/acp/acp_disallow.php
index 41ca1ca276..adb7270332 100644
--- a/phpBB/includes/acp/acp_disallow.php
+++ b/phpBB/includes/acp/acp_disallow.php
@@ -43,6 +43,8 @@ class acp_disallow
 			$sql = 'INSERT INTO ' . DISALLOW_TABLE . ' ' . $db->sql_build_array('INSERT', array('disallow_username' => $disallowed_user));
 			$db->sql_query($sql);
 
+			$cache->destroy('disallowed_usernames');
+
 			$message = $user->lang['DISALLOW_SUCCESSFUL'];
 			add_log('admin', 'LOG_DISALLOW_ADD', str_replace('%', '*', $disallowed_user));
 
@@ -61,6 +63,8 @@ class acp_disallow
 				WHERE disallow_id = ' . $disallowed_id;
 			$db->sql_query($sql);
 
+			$cache->destroy('disallowed_usernames');
+
 			add_log('admin', 'LOG_DISALLOW_DELETE');
 
 			trigger_error($user->lang['DISALLOWED_DELETED'] . adm_back_link($this->u_action), E_USER_WARNING);
diff --git a/phpBB/includes/cache.php b/phpBB/includes/cache.php
index 8d6e17a8df..01dcf5b722 100644
--- a/phpBB/includes/cache.php
+++ b/phpBB/includes/cache.php
@@ -354,6 +354,29 @@ class cache extends acm
 
 		return $parsed_items;
 	}
+
+	function obtain_disallowed_usernames(&$usernames)
+	{
+		if (($usernames = $this->get('disallowed_usernames')) === false)
+		{
+			global $db;
+
+			$sql = 'SELECT disallow_username
+				FROM  ' . DISALLOW_TABLE;
+			$result = $db->sql_query($sql);
+
+			$usernames = array();
+			while ($row = $db->sql_fetchrow($result))
+			{
+				$usernames[] = utf8_clean_string(str_replace('%', '.*?', preg_quote($row['disallow_username'], '$#')));
+			}
+			$db->sql_freeresult($result);
+
+			$this->put('disallowed_usernames', $usernames);
+		}
+
+		return true;
+	}
 }
 
 ?>
\ No newline at end of file
diff --git a/phpBB/includes/functions_user.php b/phpBB/includes/functions_user.php
index 56b92f850b..a1bdec1695 100644
--- a/phpBB/includes/functions_user.php
+++ b/phpBB/includes/functions_user.php
@@ -1120,9 +1120,11 @@ function validate_match($string, $optional = false, $match)
 */
 function validate_username($username)
 {
-	global $config, $db, $user;
+	global $config, $db, $user, $cache;
 
-	if (utf8_clean_string($user->data['username']) == utf8_clean_string($username))
+	$clean_username = utf8_clean_string($username);
+
+	if (utf8_clean_string($user->data['username']) == $clean_username)
 	{
 		return false;
 	}
@@ -1134,7 +1136,7 @@ function validate_username($username)
 
 	$sql = 'SELECT username
 		FROM ' . USERS_TABLE . "
-		WHERE username_clean = '" . $db->sql_escape(utf8_clean_string($username)) . "'";
+		WHERE username_clean = '" . $db->sql_escape($clean_username) . "'";
 	$result = $db->sql_query($sql);
 	$row = $db->sql_fetchrow($result);
 	$db->sql_freeresult($result);
@@ -1156,19 +1158,17 @@ function validate_username($username)
 		return 'USERNAME_TAKEN';
 	}
 
-	$sql = 'SELECT disallow_username
-		FROM ' . DISALLOW_TABLE;
-	$result = $db->sql_query($sql);
 
-	while ($row = $db->sql_fetchrow($result))
+	$bad_usernames = array();
+	$cache->obtain_disallowed_usernames($bad_usernames);
+
+	foreach ($bad_usernames as $bad_username)
 	{
-		if (preg_match('#^' . str_replace('%', '.*?', preg_quote($row['disallow_username'], '$#')) . '#i', $username))
+		if (preg_match('#^' . $bad_username . '#', $clean_username))
 		{
-			$db->sql_freeresult($result);
 			return 'USERNAME_DISALLOWED';
 		}
 	}
-	$db->sql_freeresult($result);
 
 	$sql = 'SELECT word
 		FROM  ' . WORDS_TABLE;