Retinitis is the commonest clinical manifestation of cytomegalovirus (CMV) infection in patients with AIDS. Untreated patients generally experience progressive involvement of the retina, which may result in blindness. Attempts to treat this condition with vidarabine and interferon-alpha have been uniformly unsuccessful. Therapy with ganciclovir (9[(1,3-dihydroxy-2-propoxy)methyl]guanine) (7.5-15 mg/[kg./d] given intravenously in two or three divided doses) has been shown to halt progression of retinitis and may result in limited healing in some cases; viremia and shedding of virus from other sites (such as the urine) are halted or reduced. However, reactivation of infection and retinitis usually occurs when ganciclovir therapy is discontinued. For that reason, most patients have received continued therapy with the drug, most commonly at a dosage of 5-6 mg/(kg.d) administered as a single intravenous infusion 5-7 days per week. Both retrospective and prospective, randomized trials have shown that maintenance therapy delays (but does not prevent) reactivation of infection. Much more limited studies with foscarnet (trisodium phosphonoformate) suggest that this drug will also be useful for treatment of CMV retinitis in AIDS patients. Further research is needed, however, since existing agents have a low therapeutic ratio and cannot be given orally to patients requiring long-term maintenance therapy.