The efficacy and safety of ganciclovir were evaluated for the treatment of 39 life-threatening or sight-threatening cytomegalovirus (CMV) infections in recipients of bone marrow transplants (15 patients), recipients of liver or renal transplants (8 patients), patients with AIDS (11 patients), and one patient each with lymphoma or systemic lupus erythematosus. Twenty-eight (72%) of 39 CMV infections improved during ganciclovir therapy, which was associated with elimination of CMV from cultures. Improvement occurred more frequently in patients with viremia, fever, and wasting (8 of 8), hepatitis (3 of 4), retinitis (5 of 5), or colitis (1 of 1), than in patients with pneumonia (11 of 21). Only two of nine marrow transplant recipients with CMV pneumonia survived, as compared with nine of 12 other immunosuppressed patients with pneumonia. However, all six marrow transplant recipients who were treated for CMV viremia, fever, and wasting without pneumonia survived. Neutropenia was the only adverse reaction associated with ganciclovir therapy and was more frequent in patients with AIDS (6 [55%] of 11) than in transplant recipients (5 [20%] of 25). These results suggest that ganciclovir is of clinical benefit for immunosuppressed patients with serious CMV infections. For bone marrow transplant recipients, ganciclovir may be more effective when used prophylactically or earlier in the course of CMV infection before the development of pneumonia.