Three additional cases of posterior interosseous-nerve paralysis secondary to rheumatoid arthritis were seen. All were initially diagnosed as extensor tendon ruptures and in one patient the extensor tendons were explored before proper diagnosis was made. The condition is a compression neuritis of the posterior interosseous nerve secondary to elbow synovitis bulging anteriorly against the overlying supinator muscle. Treatment by injection of steroids was successful in one patient; synovectomy cured the condition in the second; and a tendon transfer was used in the third.