Seventy-three patients with rheumatoid arthritis had tenosynovitis of the tendon sheath of the extensors of the fingers (ninety-three hands). Forty-one hands had ruptures of extensor tendons which were repaired at operation in addition to the dorsal tenosynovectomy. The hands in which dorsal tenosynovectomy was done prophylactically showed no rupture except in two in which errors in a Darrach procedure accompanying the tenosynovectomy allowed attrition of tendons on the roughened ulnar stump. Our present indications for prophylactic tenosynovectomy are: (1) persistent dorsal tenosynovitis despite adequate medical treatment; (2) extensor tendon rupture; and (3) recurrent dorsal tenosynovitis. Tendon rupture is considered an urgent indication for operation because further ruptures may thus be prevented.