In order to investigate the contribution of transfusion to CMV infection, 20 CMV-seropositive and 3 seronegative patients undergoing allogeneic bone marrow transplantation were studied for their CMV infection. All the patients received cellular blood component transfusion through a leukocyte depletion filter. CMV infection is defined as positive results with a nested double polymerase chain reaction (PCR) and/or IgM antibody testing. Blood samples of 10 healthy marrow donors and 10 pretransplant patients were available for PCR examination. No CMV-DNA was demonstrated. In contrast, among a total of 122 peripheral blood specimens from the 23 posttransplant patients, 51 from 14 patients were positive for CMV-DNA. Of 20 seropositive patients, 15 were transfused with CMV-seronegative blood which is consisted of 14.3% of blood donors and 5 were transfused with CMV-unscreened blood. Four (80%) of the latter 5 seropositive patients showed CMV infection. Of 15 seropositive patients transfused with CMV-seronegative blood, 11 (73.3%) exhibited CMV infection. There was no significant difference in the incidence of CMV infection between the two seropositive patient groups (Fisher's exact probability test). In most cases, CMV infection occurred after 30 days post BMT. Interstitial pneumonia developed in two seropositive patients who had received CMV-seronegative blood products, and had positive results on both PCR and IgM testing. Regardless of transfusing CMV-unscreened or seronegative blood, none of the seronegative patients were infected with CMV. Our present observations suggest that the CMV-seronegative blood component contributes little if any to prevent seropositive patients from CMV infection.