Two hundred and twenty-nine patients with chronic myeloid leukaemia in chronic phase awaiting bone marrow transplantation from an HLA-identical sibling donor were randomized as part of their conditioning, to receive splenic irradiation (SI+, 115 patients), or not (SI-, 114 patients). Both groups were identical in regard to age, sex, donor/recipient sex combination and disease activity. Survival, leukaemia-free survival, incidence of transplant-related mortality, acute and chronic graft versus host disease, incidence of rejection and probability of relapse were not different in either groups at a median follow-up time of 4.5 years (minimum follow-up 2 years). Recovery of peripheral white blood cell counts to 1 x 10(9)/l but not of platelet counts to 50 x 10(9)/l was significantly faster in patients with SI+ (21 vs 24 days). This small benefit does not justify routine splenic irradiation prior to BMT, in CML.