In a study of 637 femoral heads removed surgically because of osteoarthritis, the wear was predominantly on the posterior half in 13.2% and was more than two-thirds posterior in 7.5%. Wear was more than two-thirds anterior in 37.8%. In 36 heads from patients with rheumatoid arthritis, the wear was predominant posterior in 27.8%. No anatomical reason was found for these marked asymmetries, which, from their position, could not possibly be the result of simple weight bearing. The finding underlines the importance of muscular action in the causation of wear.