The two conditions that give rise to osteoarthritis of the hip are dysplasia and nondysplasia. Dysplasia, commonly associated with anterolateral acetabular deficiency, may lead to osteoarthritis in 40% of patients in the United States with this condition. In a distinct category of patients with so-called idiopathic arthritis, there is no apparently identifiable cause for osteoarthritis. There is emerging evidence that subtle morphologic abnormalities around the hip, resulting in femoroacetabular impingement, may often be a contributing factor to osteoarthritis in young patients.