Osteonecrosis is a common complication of renal transplantation and is an obstacle to rehabilitation. While prednisone has long been implicated as causative, the sporadic nature of aseptic necrosis (ASN) is poorly understood. Four hundred forty-four patients received kidney transplants (TX) between January 1978 and December 1984. Fifty-two patients (16%) have developed ASN. This retrospective study was developed in an attempt to define potential etiologic factors. Age, sex, donor, source, primary renal disease, duration of dialysis, and pre-TX parathyroidectomy did not correlate with ASN. Prednisone administration and duration of use prior to transplantation was equally common in ASN and control patients. Black recipients had a twofold frequency of ASN, although this association did not quite reach significance (p = 0.07). Pretransplant x-ray evidence of either osteopenia or renal osteodystrophy was significant (p less than 0.01) and apparent in 23 of 52 patients (44%). ASN was associated with a history of early acute rejection (p less than 0.02), higher final serum creatinine (p = 0.07), and greater mean prednisone (p less than 0.0001). The mean linear trend of daily prednisone dose was also significant (p less than 0.03). This study suggests that ASN is three times more likely to occur if radiographic evidence is apparent before TX. Rejection and higher daily steroid dosage correlate with a greater incidence of ASN, and blacks appear to be at greater risk.