Eleven patients with hypoplasia and partial or complete aplasia of the ulna (examples of a complex spectrum of postaxial forearm and hand abnormalities) were reviewed. Three types of ulnar deformity were observed: (1) hypoplasia, (2) partial aplasia (ossification of the proximal part of the ulna present at birth); and (3) total aplasia (ossification not development). The roentgenographically "absent" segment of the ulna may be a large fibrocartilaginous anlage attached distally to the distal radial epiphysis or the ulnar side of the carpus, or both. The tethering effect of this band may cause ulnar deviation of the wrist (and hand) and dislocation of th badial head in utero as well as progression of these deformities after birth. Resection of the distal end of the fibrocartilaginous anlage during the first to second year of life is recommended, since the results of this procedure suggest that it reduces the angular growth deformities. It is also suggested that if the one-bone-forearm operation is indicated, it should be deferred until a later age, since complications may be less likely to occur then than at the time that the anlage is resected.