A definition and classification of primary OA hip is presented based on radiological criteria. Most cases of unilateral idiopathic OA hip remain unilateral over the years and the majority of these show a distinct radiological pattern with articular cartilage loss in the superolateral area of the joint. In contrast, OA hip with loss of cartilage in the medial area is usually bilateral. A supermedial form appears to be an intermediate type, but is less well defined.It is suggested that superolateral and medial forms of OA hip are fundamentally different in nature. Leg-length disparity, to be discussed in a subsequent section, is related to the superolateral type.