Semimembranosus tenosynovitis is a common knee problem in the over-50 age-group, occurring mainly in women. It affects the reflected portion of the tendon of the semimembranosus muscle as well as the bursa below it. The tendon becomes inflamed as a result of friction at the entrance to the bony canal (semimembranosus groove), especially if osteophytes are present on the edges of the groove. Conservative treatment includes anti-inflammatory drugs, ultrasound, and friction massage. If necessary, nonresponders may be given local injection of 40-80 mg methylprednisolone acetate with 1% Xylocaine. Between 1979 and 1983 we operated on 16 patients who had not obtained relief even after three or four injections. The approach in each case was through a posterior medial oblique incision. The semimembranosus osteofibrotic tunnel was revealed and the fibrous sheath surrounding the tendon was excised. Good results were obtained in eight patients in whom the inflammatory process had been localized to only the semimembranosus insertion area: eight patients in whom the knee joint or the pes anserinus insertion was also involved achieved fair or poor results. The operation is not advised for young athletes because of the important function of the reflected head of the semimembranosus muscle.