Summary. Thirty-six patients received allogeneic (34) or syngeneic (two) bone marrow transplants as treatment for severe aplastic anaemia or acute leukaemia. Nineteen of the allogeneic recipients received methotrexate (MTX) and 15 received cyclosporin A (CyA) as the predominant immunosuppressive agent to minimize graft-versus-host disease (GVHD) post transplant. In the first 100 d post transplant renal dysfunction was much less frequent in the MTX recipients than in the CyA recipients who exhibited three distinct syndromes of nephrotoxicity: most commonly, CyA recipients developed asymptomatic azotaemia, proteinuria, urinary casts, impaired urinary concentrating ability and hypertension. Secondly, two CyA recipients developed acute reversible renal failure precipitated by systemic bacterial infection which required dialysis and in which the kidney was the sole target organ; thirdly, two recipients of HLA-genotypically non-identical grafts developed a rapidly progressive fatal syndrome with multiple organ involvement including lung, brain and kidney which clinically and histologically resembled thrombotic thrombocytopenic purpura.