In eight patients with so-called idiopathic degenerative arthritis of the hip the acetabular labrum, at surgery, was found to lie in the articulation between the femoral head and the acetabulum. In none of the patients was there a history or roentgenographic evidence of congenital dysplasia or congenital dislocation. None of the patients had had a closed reduction or manipulation of the hip during infancy, childhood, or adolescence. No patient had had a traumatic dislocation. The findings suggest that the intra-articular labrum was a developmental abnormality and we postulate that this abnormality was the cause of the degenerative arthritis.