Eighteen immunocompromised patients with cytomegalovirus (CMV) retinitis were treated with ganciclovir, an investigational antiviral drug. CMV retinitis in association with acquired immune deficiency syndrome (AIDS) developed in 17 patients; CMV retinitis developed in one patient after cardiac transplantation. Fourteen patients responded to ganciclovir treatment with improvement in CMV retinitis. In 11 patients, the response was classified as complete; in three patients, the response was partial. Continued improvement in the retinitis was often seen while the patient was on maintenance treatment. Maintenance ganciclovir therapy was required; relapse occurred in five of seven patients in whom ganciclovir treatment was interrupted. The major limiting toxic side effect of ganciclovir was neutropenia, which necessitated temporary discontinuation of ganciclovir in five patients but was reversible in all cases. Ganciclovir appears to be an effective therapy for CMV retinitis, but chronic maintenance therapy is required.