The cases of three brothers with similar lesions of osteochondritis dissecans in the knee joint are reported. The theories of the etiology and pathogenesis of this condition are reviewed and discussed. It is suggested that there are two distinct groups or types of cases. One is the type in which trauma is the cause. The second type, which occurs in children and in which the lesions are often multiple, sometimes is familial and is occasionally associated with osteochondritis in other joints, probably due to some systemic condition. It is further postulated that this may well be a metabolic disturbance. The treatment of osteochondritis dissecans in the knee is also discussed.