In five immature macaque monkeys, the right radial head was excised and the left forearm served as a control. Growth of each radius, ulna, and proximal and distal physis was followed for eight years. Total radial growth was slightly less on the side that was operated on than it was on the control side. After excision of the radial head, 95 per cent of radial growth came from the distal physis, compared with 71 per cent on the control side. A regenerated radial head, which had an irregular surface of fibrocartilaginous tissue, accounted for the remainder of growth on the side that had been operated on. Incongruous contact of articular surfaces resulted in degenerative changes in the capitellum. The arms that had been operated on were left with a flexion contracture that was an average of 12 degrees greater than that of the normal elbows, and they had a carrying angle that was an average of 6 degrees greater than that on the normal side. The arc of pronation and supination was decreased for the forearms that had been operated on, but motion of the wrist was unaffected. In our study, excision of the radial head in growing monkeys resulted in minimum radial shortening, slight deformity of the elbow, and moderate impairment of rotation of the forearm.