diff --git a/includes/module.inc b/includes/module.inc
index 3a019f2..df62fd3 100644
--- a/includes/module.inc
+++ b/includes/module.inc
@@ -425,6 +425,8 @@ function module_enable($module_list, $enable_dependencies = TRUE) {
       registry_update();
       // Refresh the schema to include it.
       drupal_get_schema(NULL, TRUE);
+      // Update the theme registry to include it.
+      drupal_theme_rebuild();
       // Clear entity cache.
       entity_info_cache_clear();
 
@@ -545,6 +547,8 @@ function module_disable($module_list, $disable_dependents = TRUE) {
     // Update the registry to remove the newly-disabled module.
     registry_update();
     _system_update_bootstrap_status();
+    // Update the theme registry to remove the newly-disabled module.
+    drupal_theme_rebuild();
   }
 
   // If there remains no more node_access module, rebuilding will be
diff --git a/includes/theme.inc b/includes/theme.inc
index 3868334..f6c931c 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -237,7 +237,7 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
 /**
  * Get the theme registry.
  *
- * @param $complete
+ * @param bool $complete
  *   Optional boolean to indicate whether to return the complete theme registry
  *   array or an instance of the ThemeRegistry class. If TRUE, the complete
  *   theme registry array will be returned. This is useful if you want to
@@ -252,7 +252,20 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
  *   class.
  */
 function theme_get_registry($complete = TRUE) {
-  static $theme_registry = array();
+  // Use the advanced drupal_static() pattern, since this is called very often.
+  static $drupal_static_fast;
+  if (!isset($drupal_static_fast)) {
+    $drupal_static_fast['registry'] = &drupal_static('theme_get_registry');
+  }
+  $theme_registry = &$drupal_static_fast['registry'];
+
+  // Initialize the theme, if this is called early in the bootstrap, or after
+  // static variables have been reset.
+  if (!is_array($theme_registry)) {
+    drupal_theme_initialize();
+    $theme_registry = array();
+  }
+
   $key = (int) $complete;
 
   if (!isset($theme_registry[$key])) {
@@ -335,6 +348,7 @@ function _theme_save_registry($theme, $registry) {
  * to add more theme hooks.
  */
 function drupal_theme_rebuild() {
+  drupal_static_reset('theme_get_registry');
   cache_clear_all('theme_registry', 'cache', TRUE);
 }
 
@@ -884,8 +898,6 @@ function list_themes($refresh = FALSE) {
  *   An HTML string representing the themed output.
  */
 function theme($hook, $variables = array()) {
-  static $hooks = NULL;
-
   // If called before all modules are loaded, we do not necessarily have a full
   // theme registry to work with, and therefore cannot process the theme
   // request properly. See also _theme_load_registry().
@@ -893,10 +905,7 @@ function theme($hook, $variables = array()) {
     throw new Exception(t('theme() may not be called until all modules are loaded.'));
   }
 
-  if (!isset($hooks)) {
-    drupal_theme_initialize();
-    $hooks = theme_get_registry(FALSE);
-  }
+  $hooks = theme_get_registry(FALSE);
 
   // If an array of hook candidates were passed, use the first one that has an
   // implementation.
diff --git a/modules/simpletest/drupal_web_test_case.php b/modules/simpletest/drupal_web_test_case.php
index 04f66ec..e0a9561 100644
--- a/modules/simpletest/drupal_web_test_case.php
+++ b/modules/simpletest/drupal_web_test_case.php
@@ -1238,6 +1238,10 @@ class DrupalWebTestCase extends DrupalTestCase {
       ->condition('test_id', $this->testId)
       ->execute();
 
+    // Reset all statics and variables to perform tests in a clean environment.
+    $conf = array();
+    drupal_static_reset();
+
     // Clone the current connection and replace the current prefix.
     $connection_info = Database::getConnectionInfo('default');
     Database::renameConnection('default', 'simpletest_original_default');
@@ -1285,10 +1289,6 @@ class DrupalWebTestCase extends DrupalTestCase {
     ini_set('log_errors', 1);
     ini_set('error_log', $public_files_directory . '/error.log');
 
-    // Reset all statics and variables to perform tests in a clean environment.
-    $conf = array();
-    drupal_static_reset();
-
     // Set the test information for use in other parts of Drupal.
     $test_info = &$GLOBALS['drupal_test_info'];
     $test_info['test_run_id'] = $this->databasePrefix;
diff --git a/modules/simpletest/tests/theme.test b/modules/simpletest/tests/theme.test
index ea0f257..641b863 100644
--- a/modules/simpletest/tests/theme.test
+++ b/modules/simpletest/tests/theme.test
@@ -101,6 +101,19 @@ class ThemeUnitTest extends DrupalWebTestCase {
     $this->drupalGet('theme-test/suggestion');
     variable_set('preprocess_css', 0);
   }
+
+  /**
+   * Ensures the theme registry is rebuilt when modules are disabled/enabled.
+   */
+  function testRegistryRebuild() {
+    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'a')), 'a', t('The theme registry contains theme_test_foo.'));
+
+    module_disable(array('theme_test'), FALSE);
+    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'b')), '', t('The theme registry does not contain theme_test_foo, because the module is disabled.'));
+
+    module_enable(array('theme_test'), FALSE);
+    $this->assertIdentical(theme('theme_test_foo', array('foo' => 'c')), 'c', t('The theme registry contains theme_test_foo again after re-enabling the module.'));
+  }
 }
 
 /**
diff --git a/modules/simpletest/tests/theme_test.module b/modules/simpletest/tests/theme_test.module
index 160d192..4b5bd08 100644
--- a/modules/simpletest/tests/theme_test.module
+++ b/modules/simpletest/tests/theme_test.module
@@ -1,6 +1,17 @@
 <?php
 
 /**
+ * Implements hook_theme().
+ */
+function theme_test_theme() {
+  return array(
+    'theme_test_foo' => array(
+      'variables' => array('foo' => NULL),
+    ),
+  );
+}
+
+/**
  * Implements hook_menu().
  */
 function theme_test_menu() {
@@ -97,6 +108,13 @@ function _theme_test_suggestion() {
 }
 
 /**
+ * Theme function for testing theme('theme_test_foo').
+ */
+function theme_theme_test_foo($variables) {
+  return $variables['foo'];
+}
+
+/**
  * Implements hook_preprocess_breadcrumb().
  *
  * Set a variable that can later be tested to see if this function ran.