We evaluated the efficacy of ganciclovir to prevent the development of cytomegalovirus interstitial pneumonitis (CMV-IP) in patients with bone marrow transplants. Of 35 patients enrolled in this study, 33 were seropositive for CMV or had seropositive donors, and two were seronegative before transplant but were positive for CMV examined by polymerase chain reaction (PCR) on days 30-37. Ganciclovir was given at a dose of 250 mg/body daily from day 30-37 to day 70. Blood, throat swabs, urine and bronchoalveolar-lavage fluid (BALF) were screened for CMV by PCR on days 30-37, 70 and 100. CVM-IP developed in two of 35 patients (5.7%) who received ganciclovir for prophylaxis, as compared with six of 39 historical controls who did not receive ganciclovir. A significant reduction of CMV detection by PCR in blood, throat swabs, and BALF was observed after administration of ganciclovir, on day 70. The incidence of neutropenia, thrombopenia and renal impairment in the study period showed no difference between the study group and the historical control. Early prophylactic use of ganciclovir appears to reduce the risk of CMV disease in allogeneic transplant recipients with positive serology or positive CMV-PCR.