The effects of radioulnar fixation were studied in 21 Quarter horse foals by applying a bone plate to the caudal aspect of the proximal part of the ulna, with screws engaging both the radius and the ulna. The plates were applied at 1 month of age in six foals (group I), 5 months of age in six foals (group II), and 7 months of age in three foals (group III). Six foals underwent sham operations at 1 month of age to serve as controls (group IV). Ulnar dysplasia and elbow subluxation developed in all treated foals. The magnitude of ulnar dysplasia was inversely related to the patient's age at fixation and was accompanied by degenerative joint disease and lameness in foals undergoing fixation at 1 and 5 months of age. Removal of the fixation appliances 16 weeks after implantation in three foals from each of groups I and II failed to reverse the degree of ulnar dysplasia. Although foals undergoing fixation at 7 months of age (group III) were not lame, radiographic evidence of subluxation and subtle degenerative changes in the articular cartilage of the treated elbow did develop. Recommendations for avoidance of radioulnar fixation were developed from these observations.