Graphical user interfaces (GUIs) are used as frontends to most of today?s software applications; testing GUIs for functional correctness is needed to ensure the overall correctness of these applications. The eventdriven nature of GUIs presents new challenges for testing. One important challenge is test suite reduction. Conventional reduction techniques/tools based on static analysis are not easily applicable due to the increased use of multi-language GUI implementations, callbacks for event handlers, virtual function calls, reflection, and multi-threading. Moreover, many existing techniques ignore event handlers from libraries, and fail to consider the context in which a handler executes. Consequently, they yield GUI test suites with seriously impaired fault-detection ability. This paper presents a new reduction technique based on the call stack coverage criterion. Call stacks may be collected for any executing program with very little overhead. An empirical study involving three large GUI-based applications shows that call stack based reduction provides an excellent tradeoff between reduction in test suite size and loss of fault-detection effectiveness.