A patient with multiple hereditary osteochondromas may have any of several severe deformities of the forearm, the most common of which are ulnar deviation of the wrist associated with relative shortening of the ulna, bowing of either or both of the bones of the forearm, shortening of the forearm, and late dislocation of the radial head. The natural history of these deformities is progression, with variable weakness, functional impairment, and cosmetic deformity of the extremity. We describe the results, after a follow-up of at least two years, in eighteen patients who underwent major surgical procedures: excision of the osteochondromas (ten patients), ulnar lengthening with excision of the osteochondromas (three patients), and ulnar lengthening with radial hemiepiphyseal stapling (seven forearms of five patients). Early excision of the osteochondromas alone did not slow the progression of the deformity. Ulnar lengthening did, on occasion, correct the ulnar drift at the wrist, but the relative shortening of the ulna recurred. Distal radial hemiepiphyseal stapling should accompany ulnar lengthening if radiocarpal angulation or subluxation of the lunate occurs with ulnar shortening. Deformities of the forearm should be treated early and aggressively to prevent disability.