Interstitial pneumonia, especially that due to cytomegalovirus (CMV), is a frequent and serious complication of allogeneic bone marrow transplantation (BMT). The influence of allogeneic BMT on the development of CMV pneumonia was investigated in comparison with that of autologous BMT. Data on 37 patients (23 allotransplants and 14 autotransplants) who survived for longer than one month were reviewed. All were conditioned for the transplant with marrow-lethal cytoreductive therapy. Interstitial pneumonia occurred in 15 allotransplant patients (65%) and in five autotransplant patients (36%), and was fatal in 10 allotransplant patients (67%) and in two autotransplant patients (40%), with no statistically significant difference between the two groups of patients. However, the high incidence of CMV pneumonia in allotransplant patients presented a striking contrast to that in autotransplant patients (13 of 23 versus one of 14, p less than 0.01). CMV pneumonia was closely associated with the occurrence of graft-versus-host disease (GVHD). Moreover, the data inferred that both delayed posttransplant immunologic recovery and numerous transfusions of blood products from multiple random donors predispose allotransplant patients to CMV pneumonia. Though combination therapy with acyclovir, interferon-alpha (IFN) and prednisolone seemed to be somewhat effective further studies are needed to verify its benefit considering that all the patients (five allotransplants and one autotransplant) who survived CMV pneumonia showed a significant seroconversion to CMV. On the other hand, neither IFN nor nonspecific gamma-globulin proved to be effective in preventing CMV pneumonia. It is thus suggested that special attention be focused not only on the development of better methods to prevent GVHD and improve posttransplant immunologic recovery but also on reduced exposure to exogenous CMV.