Since treatment of a dislocated radial head is difficult in the growing child, we looked for risk factors associated with dislocated radial head in a group of patients with forearm osteochondromas. This is a retrospective study of 21 patients with a mean age of 7.7 years. We measured forearm bone length, size of the osteochondromas, ulnar variance at the wrist, radial articular angle and determined radial head localization. The severity of ulnar shortening was associated with radial head localization (p < 0.005). Ulnar shortening caused by ulnar osteochondromas larger than 0.20 of total ulnar length was associated with radial head dislocation (p < 0.001). Early treatment of ulnar shortening in this group might prevent the radial head from dislocating.