diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9acca14..7a52326 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.3.0
+
+- Add a closed state to `Mailbox`.
+
 ## 0.2.0
 
 - Lower SDK lower bound to 3.0.0.
diff --git a/lib/mailbox.dart b/lib/mailbox.dart
index 0871a7c..ae0ff47 100644
--- a/lib/mailbox.dart
+++ b/lib/mailbox.dart
@@ -47,6 +47,7 @@ class Mailbox {
 
   static const _stateEmpty = 0;
   static const _stateFull = 1;
+  static const _stateClosed = 2;
 
   static final finalizer = Finalizer((Pointer<_MailboxRepr> mailbox) {
     calloc.free(mailbox.ref.buffer);
@@ -72,7 +73,7 @@ class Mailbox {
     final buffer = message.isEmpty ? nullptr : _toBuffer(message);
     _mutex.runLocked(() {
       if (_mailbox.ref.state != _stateEmpty) {
-        throw StateError('Mailbox is full');
+        throw StateError('Mailbox is closed or full');
       }
 
       _mailbox.ref.state = _stateFull;
@@ -83,15 +84,36 @@ class Mailbox {
     });
   }
 
+  /// Close a mailbox.
+  ///
+  /// If mailbox already contains a message then it will be dropped.
+  void close() => _mutex.runLocked(() {
+        if (_mailbox.ref.state == _stateFull) {
+          if (_mailbox.ref.bufferLength > 0) {
+            malloc.free(_mailbox.ref.buffer);
+          }
+        }
+
+        _mailbox.ref.state = _stateClosed;
+        _mailbox.ref.buffer = nullptr;
+        _mailbox.ref.bufferLength = 0;
+
+        _condVar.notify();
+      });
+
   /// Take a message from the mailbox.
   ///
   /// If mailbox is empty this will synchronously block until message
   /// is available.
   Uint8List take() => _mutex.runLocked(() {
-        while (_mailbox.ref.state != _stateFull) {
+        while (_mailbox.ref.state == _stateEmpty) {
           _condVar.wait(_mutex);
         }
 
+        if (_mailbox.ref.state == _stateClosed) {
+          throw StateError('Mailbox is closed');
+        }
+
         final result = _toList(_mailbox.ref.buffer, _mailbox.ref.bufferLength);
 
         _mailbox.ref.state = _stateEmpty;
diff --git a/pubspec.yaml b/pubspec.yaml
index e16317f..bf93c3f 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,6 +1,6 @@
 name: native_synchronization
 description: Low level synchronization primitives built on dart:ffi.
-version: 0.2.0
+version: 0.3.0
 repository: https://github.com/dart-lang/native_synchronization
 
 environment:
diff --git a/test/mailbox_test.dart b/test/mailbox_test.dart
index 2400d56..9b32bfa 100644
--- a/test/mailbox_test.dart
+++ b/test/mailbox_test.dart
@@ -29,4 +29,25 @@ void main() {
     expect(value[41], equals(42));
     expect(await helperResult, equals('success'));
   });
+
+  Future<String> startHelperIsolateClose(Sendable<Mailbox> sendableMailbox) {
+    return Isolate.run(() {
+      sleep(const Duration(milliseconds: 500));
+      final mailbox = sendableMailbox.materialize();
+      try {
+        mailbox.take();
+      } catch (_) {
+        return 'success';
+      }
+      return 'failed';
+    });
+  }
+
+  test('mailbox close', () async {
+    final mailbox = Mailbox();
+    mailbox.put(Uint8List(42)..[41] = 42);
+    mailbox.close();
+    final helperResult = startHelperIsolateClose(mailbox.asSendable);
+    expect(await helperResult, equals('success'));
+  });
 }