A case of bilateral forearm localization of multiple hereditary osteochondromata and unilateral Kienböck's disease is reported. Ulnar minus variance is frequent in both diseases. Carpal slip is often found in multiple hereditary osteochondromata. In this case, the extremity having both multiple hereditary osteochondromata and Kienböck's disease had no carpal slip. This might have produced an excess load on the lunate, which might have provoked Kienböck's disease.