A 28-year-old man with lymphoblastic lymphoma received G-CSF mobilized stem cells from his HLA identical sister, who had been taking methotrexate for psoriasis until 1 month prior to harvest. The recipient's blood group was A Rh D positive and donor's group O Rh D positive. Engraftment and major haemolysis were evident by day 9. From day 9 to day 13 he received 17 units of blood (approximately 3 L of red cells) at a time when his calculated red cell volume was 1 L. This massive transfusion requirement was not explained by his clinical condition and led us to consider factors that may have influenced the degree of haemolysis. The stem cell graft contained 2.85 x 10(6) CD34+ cells kg(-1) and we speculate there was B cell hyperactivity following the withdrawal of methotrexate in the donor and this went unchecked by the omission of methotrexate in the GVHD prophylaxis of the recipient. We have also considered the phenomenon of bystander haemolysis, previously unreported in this situation, as haemolysis of transfused group O blood must have also occurred. The case also illustrates the importance of transfusing donor type red cells and recipient type fresh frozen plasma (FFP) and platelets into minor mismatched transplant patients. The decision to revert to donor type FFP and platelets should only be made when the direct antiglobulin test is negative and the appropriate isohaemagglutinins are no longer demonstrable.