Thrombotic microangiopathy (TMA) is a well-recognised disorder which may occur in up to 6% of patients following bone marrow transplantation (BMT). Reported cases of post-BMT TMA vary widely in their reported clinical features, severity and response to therapy. Several factors are important in the aetiology, including cyclosporin A (CsA), graft-versus-host disease, irradiation, intensive conditioning chemotherapy and infection. A unifying pathogenetic mechanism is suggested, wherein these factors may interact to produce post-BMT TMA. On the basis of differences in clinical features and prognosis, we propose the classification of post-BMT TMA into four distinctive although overlapping subtypes: multifactorial fulminant TMA, conditioning-associated haemolytic-uraemic syndrome, CsA-associated nephrotoxicity with microangiopathic haemolytic anaemia (MAHA) and CsA-associated neurotoxicity with MAHA. Treatment of post-BMT TMA, especially of the poor-prognosis multifactorial fulminant subtype, is currently unsatisfactory, although occasional cases may respond to plasma exchange.