The effects of high doses of polyvalent intravenous immune globulin given for prophylaxis of cytomegalovirus infection and interstitial pneumonia in recipients of allogeneic marrow transplants were evaluated in a randomized controlled trial. Both symptomatic cytomegalovirus infection (21% compared with 46%, p = 0.03) and interstitial pneumonia (18% compared with 46%, p = 0.02) occurred less frequently in the recipients of intravenous immune globulin than in control patients. Prophylactic intravenous immune globulin was also associated with a lower incidence of graft-versus-host disease (34% in recipients compared with 65% in controls, p = 0.01), but its reduction in rates of interstitial pneumonia was independent of graft-versus-host disease and occurred in both patients with and without graft-versus-host disease. The high doses of immune globulin were well tolerated. Prophylactic intravenous immune globulin can modify the severity of cytomegalovirus infection and prevent interstitial pneumonia and possibly graft-versus-host disease in patients having allogeneic marrow transplantation.