Cytomegalovirus (CMV) seropositivity in recipients of an allogeneic bone marrow transplant (BMT) is a major risk factor for development of post-transplant CMV infection. CMV serology was assessed in 98 patients during intensive chemotherapy for haematological malignancy prior to allogeneic BMT. Thirty-seven patients eventually received a BMT; the remaining 61 patients were treated with chemotherapy alone. The proportion of seropositive patients in the BMT group increased from 36% to 48% between presentation and transplantation. This represents an increase in recipient seropositivity of 33% as a direct result of pre-transplant therapy. Mean time to seroconversion was 186 days. Seropositivity in patients receiving chemotherapy only increased from 43% to 56% during treatment and follow-up. The most likely source of the CMV acquired by these patients is CMV-infected blood products. We suggest that, wherever possible, CMV-negative blood products should be used exclusively from presentation to support all patients receiving chemotherapy in whom BMT is a therapeutic option.