The occurrence of aseptic hip necrosis was investigated in 546 renal transplant patients (639 transplants) with graft survival for a minimum of 12 months. Aseptic hip necrosis developed in 39 hips in 29 patients (5.3%) from 3 to 121 months (mean 22 months) after the renal transplantation. There was no sex-related difference in incidence of hip necrosis. The complication was significantly more common among patients younger than 20 than among those older than 40 years. The development of hip necrosis did not correlate with type of renal disease, origin of graft (cadaver of living relative donor), side of transplant or length of dialysis treatment. The number of transplantations per patient did not influence the occurrence of hip necrosis. Analyses of serum concentrations of creatinine, calcium, phosphorus and parathormone before and at different periods after transplantation revealed no patterns predictive of hip necrosis. The pathogenesis of aseptic necrosis of the femoral head obviously is multifactorial.