To elucidate whether a relationship existed between bone marrow donor cytomegalovirus (CMV) immune status and the probability of staying in remission after transplantation, a retrospective multicentre analysis was performed in 69 patients who received allogeneic bone marrow transplantation during relapse or second remission of AML or second remission of ALL. None of 12 AML patients with CMV seropositive donors had posttransplant relapse, in contrast to 7 of 10 AML patients with seronega‐tive donors. Kaplan‐Meier estimates of the 2‐yr probability of staying in remission for the two groups were 100% and 0%, respectively (p < 0.0005). This effect was independent of disease stage, donor and recipient age, recipient pretransplant CMV immune status and the occurrence of posttransplant CMV infection in recipients, and was not mediated through an increased occurrence of overt graft‐versus‐host disease (GvHD) in recipients with CMV seropositive donors. The increased probability of staying in remission was associated with an increased probability of 3‐yr disease‐free survival (p < 0.01). No similar effect was observed in patients with ALL. This study may suggest an allograft‐versus‐leukaemia effect in AML, associated with CMV seropositive donors, which seems separate from GvHD and independent of the occurrence of posttransplant CMV infection.