When an arthroplasty is needed to treat osteonecrosis of the femoral head, the use of a component that replaces or resurfaces only the femoral head is often considered as an alternative to total hip replacement if the acetabulum appears radiographically normal. However, the long-term results of the use of endoprostheses have often been poor, secondary in part to progressive degeneration of the acetabular cartilage and to protrusio acetabuli. To help to explain these observations, we examined the acetabular cartilage in forty-one hips in which a primary total hip replacement had been performed because of osteonecrosis of the femoral head in association with a radiographically normal acetabulum. The cartilage in the superior, weight-bearing region of the acetabulum was grossly abnormal in forty of the forty-one hips and it was histologically abnormal in all thirty-three hips that were so evaluated. In all but one hip, gross degeneration of the cartilage was apparent, involving less than 20 percent of the acetabulum; the degeneration was graded as mild (superficial fibrillation and slight irregularity of the surface) in sixteen hips, moderate (moderate fibrillation, alteration in color and consistency, and thinning of cartilage without complete erosion to bone) in twenty hips, and severe (marked fibrillation, alteration in color and consistency, and marked thinning of cartilage with areas of complete erosion to bone) in four hips. These observations emphasize the fact that radiographs cannot demonstrate early degeneration of cartilage and that, by the time that an arthroplasty is needed, degenerative changes are already present in the acetabular cartilage of a high percentage of hips with osteonecrosis, even when radiographs of the acetabulum show no abnormalities. These findings should be kept in mind when a decision is being made regarding which type of arthroplasty should be done in a patient who has osteonecrosis of the femoral head and regarding when to do the procedure.