We retrospectively evaluated the results after ulnar lengthening and radial deformity correction using an external fixator for forearm deformities caused by osteochondromas. Eight forearms were treated surgically in seven patients with multiple hereditary osteochondroma. The mean follow-up time was 40 months (range, 20 to 60 months). The average radial articular angle improved from 43 degrees to 35.5 degrees (range, 28 to 56 degrees) and the carpal slip improved from 69.5% to 55% (range, 40 to 60%) postoperatively. The average shortening of the ulna was reduced from 2.06 cm to 0.44 cm (range 0 to 1 cm) after the treatment. There were no serious complications associated with the surgery; two minor pin track infections were successfully treated by local wound care and antibiotics. Although technically demanding, ulnar osteotomy and gradual lengthening by an external fixator provided promising results in the treatment of forearm deformities in children with multiple osteochondroma.