A case of thrombotic thrombocytopenic purpura (TTP) is reported in a 40-year-old man 6 months after allogeneic bone marrow transplantation for multiple myeloma. The features of TTP included microangiopathic haemolytic anaemia, severe thrombocytopenia, fluctuating neurological abnormalities, and progressive renal impairment. Despite treatment with anti-platelet agents, prostacyclin infusion, intensive immunosuppression and prolonged plasma exchange, the patient developed end-stage renal failure and is now on maintenance haemodialysis 18 months after the onset of TTP. Graft-versus-host disease and cytomegalovirus infection could not be implicated as aetiological factors, and cyclosporin medication had ceased 1 week before the clinical onset of his disease. The unusually intensive pre-transplant chemotherapy and radiotherapy protocol used in this patient appear to be most likely cause of the generalized endothelial damage resulting in TTP in this patient.