New cases of osteonecrosis of the femoral head in the United States number between 10,000 and 20,000 per year. This disease usually affects patients in their late 30s and early 40s. Although a number of authors have related specific risk factors to this disease, its etiology, pathogenesis, and treatment remain a source of considerable controversy. This disorder has been associated with corticosteroid use, substance abuse, and various systemic medical conditions. Either direct damage to osteocytes (e.g., by toxin production) or indirect damage (e.g., due to disorders in fat metabolism or hypoxia) may lead to osteonecrosis. Patients at increased risk for osteonecrosis should be monitored closely. Unfortunately, most cases are diagnosed in an advanced stage of disease, when minimally invasive surgical procedures are no longer helpful. Furthermore, patients in the advanced stage of the disease must undergo total hip replacement at a young age, which carries a poor long-term prognosis.