Nontraumatic avascular necrosis of the femoral head in the adult is being diagnosed with increasing frequency. Predisposing factors can be identified in approximately 80% of the cases and help lead to an early diagnosis. In most instances the diagnosis will be confirmed by good quality anteroposterior and lateral radiographs of the hip. If these are nondiagnostic, magnetic resonance imaging studies should be performed. These are currently extremely sensitive and specific for the diagnosis of avascular necrosis. Except in the case of a very small asymptomatic lesion, conservative or nonoperative management is ineffective in most cases, and early prophylactic surgery should be performed to prevent collapse of the femoral head. The use of an effective, quantitative method for evaluation and staging will help to determine the prognosis and the specific method of treatment. Prophylactic procedures include core decompression, various types of osteotomies, and bone grafting procedures. Use of a fibular graft with microvascular anastomosis is technically demanding, but is producing gratifying results in the hands of a limited number of investigators. If the hip disease has progressed to the point of significant collapse with pain and disability, prophylactic procedures are generally of little avail. Patients should be treated symptomatically until such time as arthroplasty is required. Although modified cup arthroplasties are receiving renewed attention, most patients will undergo either endoprosthetic replacement or total hip replacement. Results with total hip replacement are generally more consistent and more durable than with an endoprosthesis, and this is currently the treatment of choice in most instances when arthroplasty is required.