The tumor mass originally appeared as a cystic swelling on the lateral aspect of the knee, and at operation was found to communicate with the knee joint and to arise from the synovial membrane in the posterior compartment of the knee. The tumor was firmly adherent to the external semilunar cartilage, which, together with the tumor mass, was removed. Eight years and five months after operation there was no evidence of recurrence, and the patient had normal function of the knee joint.