The division of osteoarthritis into primary and secondary varieties implies that these are aetiologically distinct entities, the former being due to some intrinsic defect of cartilage and the latter resulting from previous articular damage. This traditional concept is questioned and the hypothesis is advanced that osteoarthritis is always secondary to some underlying abnormality of the joint. A detailed clinical, radiographic and morbid anatomical study of 327 cases of osteoarthritis of the hip is presented. In all but twenty-seven some predisposing abnormality of the joint was diagnosed: 107 (33%) were associated with major pathology such as Perthes' disease or epiphysiolysis; minor acetabular dysplasia was present in sixty-seven (20%), with a male: female ratio of 1:10; minimal femoral head tilt was demonstrated in fifty-nine (18%), the male: female ratio being 14:1; and in forty-three (13%) there were features suggesting an underlying inflammatory arthritis. On the basis of this study a new classification is proposed and osteoarthritis of the hip is divided into three pathogenetic groups: 1) failure of essentially normal cartilage subjected to abnormal or incongruous loading for long periods; 2) damaged or defective cartilage failing under normal conditions of loading; 3) break-up of articular cartilage due to defective subchondral bone.