Deformity of the forearm is common in patients with hereditary multiple osteochondromas. It produces cosmetic and functional impairment. The deformity of the forearm in hereditary multiple osteochondromas that we described is often associated with osteochondroma of the distal part of ulna causing ulnar longitudinal hypoplasia. This osteochondroma mechanically disturbs the axial alignment of the adjacent joints, either directly or though tension on the interosseous membrane. We reviewed 9 cases of forearm deformity caused by hereditary multiple osteochondromas in 9 patients, in whom ulnar lengthening with excision of osteochondroma was a main procduree. The patients were followed along for a mean of 3 year 6 months. The procedures included ulnar lengthening with excision of osteochondroma in the distal part of the ulna in 6 patients, ulnar lengthening with excision of the osteochondroma and corrective osteotomy of the radius in 2 patients, and stapling of the distal physis of the radius with excision of osteochondroma in 1 patient. Ulnar lengthening was performed by an immediate bone graft and internal fixation in 5 patients, or gradual distraction with and external fixator in 3 patients. The cosmetic results of the surgery were very gratifying and the range of motion of radial deviation at the wrist was increased. Partial recurrence of the deformity was seen during follow up in the skeletally immature patients. However, in general, Ulnar lengthening with excision of osteochondroma was an effective procedure to restore cosmesis as well as function of the wrist.