The current study evaluated the prevalence of anticardiolipin antibodies, which have been associated with thrombotic phenomena, in patients with nontraumatic osteonecrosis of the hip and assessed whether the presence of such antibodies is associated with an increased risk for the development of bone necrosis. Forty consecutive patients (25 men and 15 women) with nontraumatic osteonecrosis of the hip were studied. Their ages ranged from 19 to 56 years (average, 34.3 years). Anticardiolipin antibodies were present in 37.5% (15 of 40) of the tested patients, a significantly higher rate than is seen in healthy subjects, of whom only one of 100 had low titer anticardiolipin antibodies (1%). Six of 40 patients tested positive for immunoglobulin M alone, and six of 40 patients tested positive for immunoglobulin A alone. Three of 40 patients tested positive for immunoglobulin M and immunoglobulin A isotype. The results of the current study indicate an increased incidence of anticardiolipin antibodies in patients with nontraumatic osteonecrosis of the femoral head, which may reflect that anticardiolipin antibodies play a role in the pathogenesis of bone necrosis by predisposing to thrombotic phenomena.