Biopsy-proven thrombotic microangiopathy (TMA) was found in 22 of 436 (5%) renal transplant recipients, with similar incidence in recipients of cadaver or living related allografts. All patients with TMA presented different degrees of severity of the hemolytic uremic syndrome (HUS). Prognosis was poor when HUS occurred shortly after transplant in recipients of cadaveric kidneys (55% graft loss). It was more favorable when HUS occurred later in the post-transplant course or in recipients with allografts from living related donors, irrespective of time of occurrence. Other factors such as extent of TMA, degree of thrombocytopenia, hemolysis or renal dysfunction were not predictive of graft loss. Cyclosporine was resumed in 14 of 16 recipients shortly after clinical recovery without recurrence of HUS. In conclusion, HUS carries poor prognosis when occurring shortly after transplant in cadaver kidney recipients. Once the graft function improves, cyclosporine can be safely resumed.