Fifteen knees in patients suffering from symptomatic osteoarthritis secondary to chronically dislocated extensor mechanisms have been treated by knee replacement. Five knees had permanently dislocated patellae while 10 knees (four of which had undergone patellectomies) had dislocations of the extensor mechanisms with knee flexion. In fourteen instances satisfactory realignment of the extensor mechanism was obtained. In twelve knees, a substantial functional improvement was achieved. Two knees had persistent pain and one had unexpected tibio-femoral instability.