To determine whether hypercholesterolemia is an associated factor for idiopathic avascular necrosis, serum cholesterol levels were measured in all patients presenting with idiopathic avascular necrosis of the femoral head from December 1987 to December 1995. Of 77 patients, with 114 affected hips, presenting with avascular necrosis of the femoral head, 58 patients, with 81 affected hips, were excluded for known associated risk factors. Of the remaining 19 patients (33 hips) who were diagnosed with idiopathic avascular necrosis, 16 had serum cholesterol levels that were statistically elevated (mean value, 254 mg/dL) compared with accepted age-matched control standards (P < 0.0031). Four of the 16 patients were taking cholesterol-lowering medications at the time of initial evaluation and testing. In addition, 12 of the 16 patients had bilateral avascular necrosis, suggesting that the systemic effect of hypercholesterolemia may be involved in the pathophysiology of osteonecrosis. These results suggest an association between elevated serum cholesterol levels and idiopathic avascular necrosis of the femoral head.