Eight cases of haemolytic uraemic syndrome occurring after bone marrow transplantation are presented and the other 15 reported cases are reviewed. Two patients were recipients of autologous marrow whereas all cases previously reported occurred after allogeneic transplantation. Six patients had not received cyclosporin and two had no evidence of cytomegalovirus infection. The roles of cyclosporin, cytomegalovirus infection, graft-versus-host disease, total body irradiation (TBI) and chemotherapeutic drugs as aetiological agents are discussed. It is postulated that TBI, perhaps potentiated by cyclophosphamide, is likely to be the most important factor but that other agents may act additively with TBI and influence the time course and severity of the disease.