diff --git a/wp-comments-post.php b/wp-comments-post.php
index b781383a5b..03d52bc04e 100644
--- a/wp-comments-post.php
+++ b/wp-comments-post.php
@@ -75,7 +75,7 @@ if ( '' == $comment_content )
 
 $comment_parent = isset($_POST['comment_parent']) ? absint($_POST['comment_parent']) : 0;
 
-$commentdata = compact('comment_post_ID', 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_content', 'comment_type', 'comment_parent', 'user_ID');
+$commentdata = compact('comment_post_ID', 'comment_author', 'comment_author_email', 'comment_author_url', 'comment_content', 'comment_type', 'comment_parent', 'user_id');
 
 $comment_id = wp_new_comment( $commentdata );
 
diff --git a/wp-includes/comment.php b/wp-includes/comment.php
index ad9d50bf39..d1623d2bd9 100644
--- a/wp-includes/comment.php
+++ b/wp-includes/comment.php
@@ -1076,7 +1076,11 @@ function wp_insert_comment($commentdata) {
  * @return array Parsed comment information.
  */
 function wp_filter_comment($commentdata) {
-	$commentdata['user_id']              = apply_filters('pre_user_id', $commentdata['user_ID']);
+	// user_id is preferred. user_ID is accepted for back-compat.
+	if ( isset($commentdata['user_ID']) )
+		$commentdata['user_id'] = $commentdata['user_ID'] = apply_filters('pre_user_id', $commentdata['user_ID']);
+	else
+		$commentdata['user_id'] = $commentdata['user_ID'] = apply_filters('pre_user_id', $commentdata['user_id']);
 	$commentdata['comment_agent']        = apply_filters('pre_comment_user_agent', $commentdata['comment_agent']);
 	$commentdata['comment_author']       = apply_filters('pre_comment_author_name', $commentdata['comment_author']);
 	$commentdata['comment_content']      = apply_filters('pre_comment_content', $commentdata['comment_content']);
@@ -1127,7 +1131,11 @@ function wp_new_comment( $commentdata ) {
 	$commentdata = apply_filters('preprocess_comment', $commentdata);
 
 	$commentdata['comment_post_ID'] = (int) $commentdata['comment_post_ID'];
-	$commentdata['user_ID']         = (int) $commentdata['user_ID'];
+	// user_id is preferred. user_ID is accepted for back-compat.
+	if ( isset($commentdata['user_ID']) )
+		$commentdata['user_id'] = $commentdata['user_ID'] = (int) $commentdata['user_ID'];
+	else
+		$commentdata['user_id'] = $commentdata['user_ID'] = (int) $commentdata['user_id'];
 
 	$commentdata['comment_parent'] = absint($commentdata['comment_parent']);
 	$parent_status = ( 0 < $commentdata['comment_parent'] ) ? wp_get_comment_status($commentdata['comment_parent']) : '';
@@ -1153,7 +1161,7 @@ function wp_new_comment( $commentdata ) {
 
 		$post = &get_post($commentdata['comment_post_ID']); // Don't notify if it's your own comment
 
-		if ( get_option('comments_notify') && $commentdata['comment_approved'] && $post->post_author != $commentdata['user_ID'] )
+		if ( get_option('comments_notify') && $commentdata['comment_approved'] && $post->post_author != $commentdata['user_id'] )
 			wp_notify_postauthor($comment_ID, $commentdata['comment_type']);
 	}
 
diff --git a/wp-includes/user.php b/wp-includes/user.php
index 7ee0ee9a1f..0554762079 100644
--- a/wp-includes/user.php
+++ b/wp-includes/user.php
@@ -451,7 +451,7 @@ function update_usermeta( $user_id, $meta_key, $meta_value ) {
  * @param int $user_id Optional. User ID to setup global data.
  */
 function setup_userdata($user_id = '') {
-	global $user_login, $userdata, $user_level, $user_ID, $user_email, $user_url, $user_pass_md5, $user_identity;
+	global $user_login, $userdata, $user_level, $user_id, $user_ID, $user_email, $user_url, $user_pass_md5, $user_identity;
 
 	if ( '' == $user_id )
 		$user = wp_get_current_user();
@@ -464,7 +464,7 @@ function setup_userdata($user_id = '') {
 	$userdata = $user->data;
 	$user_login	= $user->user_login;
 	$user_level	= (int) isset($user->user_level) ? $user->user_level : 0;
-	$user_ID	= (int) $user->ID;
+	$user_id = $user_ID = (int) $user->ID;
 	$user_email	= $user->user_email;
 	$user_url	= $user->user_url;
 	$user_pass_md5	= md5($user->user_pass);