The association of granulocyte transfusions with cytomegalovirus infection and interstitial pneumonitis in allogeneic marrow transplant recipients was reviewed. Data from 387 patients with aplastic anemia or acute leukemia receiving transplants from human leukocyte antigen-matched siblings were analyzed. Acquisition of cytomegalovirus infection was higher in recipients of prophylactic and therapeutic granulocytes than in control subjects. The incidence was significantly different, however, only in cytomegalovirus seronegative patients who received granulocytes from seropositive granulocyte donors. There was no significant increase in the total incidence of or mortality from all forms of interstitial pneumonitis.