diff --git a/CHANGELOG.md b/CHANGELOG.md
index b56e7d1b..410817a0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,8 @@
 
 * Don't emit loud comments in functions.
 
+* Detect import loops.
+
 * Fix `@import` with a `supports()` clause.
 
 * Forbid functions named "and", "or", and "not".
diff --git a/lib/src/visitor/perform.dart b/lib/src/visitor/perform.dart
index 03c76f0a..de1becf8 100644
--- a/lib/src/visitor/perform.dart
+++ b/lib/src/visitor/perform.dart
@@ -121,6 +121,8 @@ class _PerformVisitor
   /// multiple times.
   final _importedFiles = <String, Stylesheet>{};
 
+  final _activeImports = new Set<Uri>();
+
   /// The extender that handles extensions for this perform run.
   final _extender = new Extender();
 
@@ -182,6 +184,7 @@ class _PerformVisitor
   }
 
   CssStylesheet run(Stylesheet node) {
+    _activeImports.add(node.span.sourceUrl);
     visitStylesheet(node);
     return _root;
   }
@@ -553,6 +556,13 @@ class _PerformVisitor
   /// Adds the stylesheet imported by [import] to the current document.
   void _visitDynamicImport(DynamicImport import) {
     var stylesheet = _loadImport(import);
+
+    var url = stylesheet.span.sourceUrl;
+    if (_activeImports.contains(url)) {
+      throw _exception("This file is already being imported.", import.span);
+    }
+
+    _activeImports.add(url);
     _withStackFrame("@import", import.span, () {
       _withEnvironment(_environment.global(), () {
         for (var statement in stylesheet.children) {
@@ -560,6 +570,7 @@ class _PerformVisitor
         }
       });
     });
+    _activeImports.remove(url);
   }
 
   /// Loads the [Stylesheet] imported by [import], or throws a