The purposes of this review are to examine the epidemiology of disease due to cytomegalovirus (CMV) in recipients of autologous and allogeneic marrow transplants and to compare different antiviral regimens used for the prevention of such disease in recipients of allogeneic marrow transplants, with an emphasis on ganciclovir. In seven studies, ganciclovir reduced the incidence of CMV infection and disease after allogeneic marrow transplantation. In one study mortality after transplantation was reduced because of a decreased rate of CMV-related death among ganciclovir-treated patients. Ganciclovir was effective when given to all CMV-seropositive patients (prophylaxis) or to patients who were considered at high risk for CMV disease on the basis of a positive surveillance culture (early treatment). The effectiveness of ganciclovir for the prevention of CMV infection and disease is limited by drug-induced neutropenia. Experience with other antiviral agents, such as foscarnet, has been limited. Initial studies of the adoptive transfer of CMV-specific CD8+ cytotoxic T cells have been conducted. In short, ganciclovir is currently effective for the prevention of CMV disease in allogeneic marrow transplant recipients, but its usefulness is limited by neutropenia. Future studies must be aimed at confining the toxicity of ganciclovir to patients at the highest risk for CMV disease.