diff --git a/includes/theme.inc b/includes/theme.inc
index ee73965..016215c 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -1151,6 +1151,13 @@ function theme($hook, $variables = array()) {
     }
   }
 
+  // Include a file if the theme function is held elsewhere.
+  if (!empty($info['includes'])) {
+    foreach ($info['includes'] as $include_file) {
+      include_once DRUPAL_ROOT . '/' . $include_file;
+    }
+  }
+
   // Generate the output using either a function or a template.
   $output = '';
   if (isset($info['function'])) {
diff --git a/modules/simpletest/tests/theme.test b/modules/simpletest/tests/theme.test
index 519a7a9..c66ffa0 100644
--- a/modules/simpletest/tests/theme.test
+++ b/modules/simpletest/tests/theme.test
@@ -68,6 +68,21 @@ class ThemeTestCase extends DrupalWebTestCase {
   }
 
   /**
+   * Ensures theme suggestions located in include files are loaded.
+   */
+  function testSuggestionsInclude() {
+    $this->drupalGet('theme-test/suggestion-include');
+    $this->assertText('Original theme function for testing suggestions stored in include files.');
+    module_enable(array('theme_suggestions_test'));
+    // Test with both an unprimed and primed theme registry.
+    drupal_theme_rebuild();
+    for ($i = 0; $i < 2; $i++) {
+      $this->drupalGet('theme-test/suggestion-include');
+      $this->assertText('Theme function loaded from include file.');
+    }
+  }
+
+  /**
    * Ensure page-front template suggestion is added when on front page.
    */
   function testFrontPageThemeSuggestion() {
diff --git a/modules/simpletest/tests/theme_suggestions_test.inc b/modules/simpletest/tests/theme_suggestions_test.inc
new file mode 100644
index 0000000..0b87bf9
--- /dev/null
+++ b/modules/simpletest/tests/theme_suggestions_test.inc
@@ -0,0 +1,5 @@
+<?php
+
+function theme_theme_suggestions_test_include($variables) {
+  return 'Theme function loaded from include file.';
+}
diff --git a/modules/simpletest/tests/theme_suggestions_test.info b/modules/simpletest/tests/theme_suggestions_test.info
new file mode 100644
index 0000000..3020dbb
--- /dev/null
+++ b/modules/simpletest/tests/theme_suggestions_test.info
@@ -0,0 +1,6 @@
+name = "Theme suggestions test"
+description = "Support module for theme hook suggestions testing."
+package = Testing
+version = VERSION
+core = 7.x
+hidden = TRUE
diff --git a/modules/simpletest/tests/theme_suggestions_test.module b/modules/simpletest/tests/theme_suggestions_test.module
new file mode 100644
index 0000000..e938d92
--- /dev/null
+++ b/modules/simpletest/tests/theme_suggestions_test.module
@@ -0,0 +1,19 @@
+<?php
+
+/**
+ * Implements hook_theme().
+ */
+function theme_suggestions_test_theme($existing, $type, $theme, $path) {
+  $items['theme_suggestions_test_include'] = array(
+    'file' => 'theme_suggestions_test.inc',
+    'variables' => array(),
+  );
+  return $items;
+}
+
+/**
+ * Preprocesses variables for theme_theme_suggestions_test_include().
+ */
+function theme_suggestions_test_preprocess_theme_test_suggestion_include(&$variables) {
+  $variables['theme_hook_suggestion'] = 'theme_suggestions_test_include';
+}
diff --git a/modules/simpletest/tests/theme_test.module b/modules/simpletest/tests/theme_test.module
index 61a12bb..504a987 100644
--- a/modules/simpletest/tests/theme_test.module
+++ b/modules/simpletest/tests/theme_test.module
@@ -17,6 +17,9 @@ function theme_test_theme($existing, $type, $theme, $path) {
   $items['theme_test_foo'] = array(
     'variables' => array('foo' => NULL),
   );
+  $items['theme_test_suggestion_include'] = array(
+    'variables' => array(),
+  );
   return $items;
 }
 
@@ -53,6 +56,12 @@ function theme_test_menu() {
     'access callback' => TRUE,
     'type' => MENU_CALLBACK,
   );
+  $items['theme-test/suggestion-include'] = array(
+    'title' => 'Suggestion include',
+    'page callback' => '_theme_test_suggestion_include',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
   return $items;
 }
 
@@ -132,3 +141,17 @@ function _theme_test_suggestion() {
 function theme_theme_test_foo($variables) {
   return $variables['foo'];
 }
+
+/**
+ * Page callback, calls a theme hook suggestion located in an include file.
+ */
+function _theme_test_suggestion_include() {
+  return array('#theme' => 'theme_test_suggestion_include');
+}
+
+/**
+ * Theme function for testing suggestions stored in include files.
+ */
+function theme_theme_test_suggestion_include($variables) {
+  return 'Original theme function for testing suggestions stored in include files.';
+}