Ganciclovir which has proved effective in the treatment of cytomegalovirus (CMV) infection was given prophylactically to 40 bone marrow transplant (BMT) patients pre and post-transplant in seropositive patients and post-transplant in seronegative patients with a seropositive donor. All patients were transfused with screened blood products and 33 received CMV hyperimmune globulin. They were compared with an historical control group consisting of 39 patients who had received significantly more unscreened blood products (p = 0.01) and less HLA-mismatched marrow transplants (p = 0.05). Toxicity of ganciclovir was hematological-neutropenia was responsible for cessation of the drug in seven patients and transfusion requirements were significantly higher in the ganciclovir group. Non-hematological toxicity did not occur in any patient. Only one patient (2.5%) experienced symptomatic CMV infection and no patient developed CMV pneumonitis. In contrast, in the control group, 23 (59%) patients had clinical symptoms of CMV infection (p < 0.0001) and 4 (10%) experienced CMV pneumonitis (p < 0.01). Ganciclovir significantly reduced the incidence of positive CMV antigenemia (7.5% in the treated group vs 72% in the control group; p < 0.01). However, ganciclovir delivery did not result in an improved overall survival due to a higher rate of regimen-related deaths and chronic GVHD mostly in patients transplanted from an HLA-mismatched donor. The prophylactic administration of ganciclovir abrogates CMV pneumonitis and considerably reduces the incidence of CMV infection in BM recipients at high risk of developing this disease after transplantation.