Cytomegalovirus (CMV) disease can be prevented by administration of ganciclovir prophylactically post-transplant. However, up to 30% of patients discontinue use of ganciclovir as a result of profound neutropenia and may subsequently develop CMV infections while unprotected. To prevent reactivation of CMV, we administered foscarnet to 39 adults unable to receive ganciclovir due to delayed engraftment or ganciclovir-induced neutropenia. Twenty-four (62%) of the patients had received T cell-depleted marrow transplants. Foscarnet sodium 60 mg/kg i.v. daily was continued until the neutropenia resolved, at which time ganciclovir was resumed. CMV prophylaxis commenced at a median of 28 days following transplantation. Median time to initiation of foscarnet was day 60 post-transplant, and the median duration of treatment was 22 days. Foscarnet was well-tolerated. Six (15%) patients had CMV detected while receiving prophylaxis, and CMV-related mortality was 5%. Foscarnet is a safe and effective agent for prevention of CMV disease in allogeneic transplant recipients unable to receive ganciclovir.