Thirty-three of 85 patients undergoing marrow transplantation between 1969 and 1973 developed interstitial pneumonia; 23 died. The clinical syndrome consisted of tachypnea, cough, fever, rales, and hypoxemia; the radiologic findings were variable. The development of interstitial pneumonia was significantly associated with graft-versus-host disease and allogenic grafting; patients with isogenic grafts were relatively spared. The increased attack rate between 1969-71 (20%) and 1972-73 (49%) was not fully explained by improved long-term survival, by an increased proportion of allogenic transplants, or by an increased incidence of graft-versus-host disease. Intranuclear inclusions typical of cytomegalovirus were identified in 9 of 17 autopsy-confirmed cases, and patients whose marrow donors had positive cytomegalovirus antibody titers developed interstitial pneumonia more often than patients whose donor had negative titers. Interstitial pneumonia is an important cause of morbidity and mortality after human marrow transplantation. No effective treatment is presently available.