The clinical and radiologic features of osteochondritis dissecans in the knee and hock joint of horses are described. The material includes 91 horses, of which 43 had the lesion in one or both knee joints, and 48 in one or both hock joints. It was found that osteochondritis dissecans of the knee joint was more common than the one in the hock joint in thoroughbreds and halfbreds, while the lesion in the hock joint was most common in standardbred trotters. In the knee the lesion was most often located to the lateral trochlear ridge. In the hock joint the predilection site was the intermediate ridge of the tibia. The lesions were of obvious clinical importance and the knee joint was usually more severely affected than the hock joint. Prognosis was also more guarded in horses with knee joint involvement, regardless whether or not surgical treatment was instituted. Arthrotomy and removal of fragments in the osteochondral defects or of loose bodies was the standard surgical procedure. It was felt that this treatment improved prognosis somewhat, particularly in the horses with hock joint lesions. It was found that there are many similarities between osteochondritis dissecans in horses and in pigs and dogs. However, further investigation is needed to shed more light on etiology and pathogenesis and on the nature of osteochondritis dissecans in the horse.