diff --git a/wp-includes/class-wp.php b/wp-includes/class-wp.php
index b1b1e2876e..c76030b32c 100644
--- a/wp-includes/class-wp.php
+++ b/wp-includes/class-wp.php
@@ -378,12 +378,29 @@ class WP {
 
 		if ( ! empty( $status ) )
 			status_header( $status );
+
+		// If Last-Modified is set to false, it should not be sent (no-cache situation).
+		if ( isset( $headers['Last-Modified'] ) && false === $headers['Last-Modified'] ) {
+			unset( $headers['Last-Modified'] );
+
+			// In PHP 5.3+, make sure we are not sending a Last-Modified header.
+			if ( function_exists( 'header_remove' ) ) {
+				@header_remove( 'Last-Modified' );
+			} else {
+				// In PHP 5.2, send an empty Last-Modified header, but only as a
+				// last resort to override a header already sent. #WP23021
+				foreach ( headers_list() as $header ) {
+					if ( 0 === stripos( $header, 'Last-Modified' ) ) {
+						$headers['Last-Modified'] = '';
+						break;
+					}
+				}
+			}
+		}
+
 		foreach( (array) $headers as $name => $field_value )
 			@header("{$name}: {$field_value}");
 
-		if ( isset( $headers['Last-Modified'] ) && empty( $headers['Last-Modified'] ) && function_exists( 'header_remove' ) )
-			@header_remove( 'Last-Modified' );
-
 		if ( $exit_required )
 			exit();
 
diff --git a/wp-includes/functions.php b/wp-includes/functions.php
index a41d32fe97..f4b4634b5a 100644
--- a/wp-includes/functions.php
+++ b/wp-includes/functions.php
@@ -902,7 +902,6 @@ function status_header( $header ) {
 function wp_get_nocache_headers() {
 	$headers = array(
 		'Expires' => 'Wed, 11 Jan 1984 05:00:00 GMT',
-		'Last-Modified' => '',
 		'Cache-Control' => 'no-cache, must-revalidate, max-age=0',
 		'Pragma' => 'no-cache',
 	);
@@ -910,6 +909,7 @@ function wp_get_nocache_headers() {
 	if ( function_exists('apply_filters') ) {
 		$headers = (array) apply_filters('nocache_headers', $headers);
 	}
+	$headers['Last-Modified'] = false;
 	return $headers;
 }
 
@@ -924,10 +924,25 @@ function wp_get_nocache_headers() {
  */
 function nocache_headers() {
 	$headers = wp_get_nocache_headers();
+
+	unset( $headers['Last-Modified'] );
+
+	// In PHP 5.3+, make sure we are not sending a Last-Modified header.
+	if ( function_exists( 'header_remove' ) ) {
+		@header_remove( 'Last-Modified' );
+	} else {
+		// In PHP 5.2, send an empty Last-Modified header, but only as a
+		// last resort to override a header already sent. #WP23021
+		foreach ( headers_list() as $header ) {
+			if ( 0 === stripos( $header, 'Last-Modified' ) ) {
+				$headers['Last-Modified'] = '';
+				break;
+			}
+		}
+	}
+
 	foreach( $headers as $name => $field_value )
 		@header("{$name}: {$field_value}");
-	if ( empty( $headers['Last-Modified'] ) && function_exists( 'header_remove' ) )
-		@header_remove( 'Last-Modified' );
 }
 
 /**