In this paper, an adaptive sliding mode control method for synchronization of a class of chaotic systems with fully unknown parameters is introduced. In this method, no knowledge of the bounds of parameters is required in advance and the parameters are updated through an adaptive control process. We use our proposed method to synchronize two chaotic gyros, which has been the subject of intense study during the recent years for its application in the navigational, aeronautical, and space engineering domains. The effectiveness of our method is demonstrated in simulation environment and the results are compared with some recent schemes proposed in the literature for the same task.