Cytomegalovirus (CMV) is a herpes virus which can give rise to primary infections, reactivated infections, or reinfections in humans. Seroepidemiologic studies have shown CMV infection to be worldwide with the highest antibody prevalences detected in Third World countries; however, significant regional variations can be seen within a given country. Antibody prevalence varies directly with age and inversely according to socioeconomic status. Numerous prospective studies of blood transfusion recipients carried out since 1966 have shown marked differences in infection rates but relatively little associated disease. Infection rates were highest in seronegative recipients given large amounts of fresh blood. Recently published reports have shown substantially lower infection rates than earlier studies, a change likely to be due to the current practice of transfusing fewer units of older blood. CMV has not been found to play a significant role in the etiology of posttransfusion hepatitis. CMV infections have been found to be an important source of morbidity and mortality in immunocompromised patients. Several studies of transfused, premature infants have shown significant differences in infection rates and disease expression. Seronegative low-birth-weight infants receiving blood from seropositive donors are at greatest risk. Blood from CMV-seronegative donors substantially lowers the risk of infection. Receiving a kidney or heart from a CMV-seropositive donor appears to be a more salient risk factor than blood transfusion in renal and cardiac transplant patients who are also more likely to have symptomatic CMV infections. Leukocyte transfusions have been found to be a significant source of CMV infection and disease in bone marrow transplant patients.(ABSTRACT TRUNCATED AT 250 WORDS)