Cytomegalovirus (CMV) retinitis is the major cause of visual loss in acquired immune deficiency syndrome (AIDS). Thirty-one patients with active CMV retinitis were treated with the new antiviral drug, Foscarnet (trisodium phosphonoformate). After a 3-week course of induction therapy, the retinitis improved in 29 of 31 patients (93.5%). Complete resolution of the retinitis was seen in 19 cases (61.3%). Ten patients had partial resolution (32.2%) and two (6.5%) failed to respond. After induction therapy, six patients were put on a low-dose maintenance regimen. All patients without maintenance therapy relapsed within 3 weeks after discontinuation of Foscarnet. The rate of relapse on maintenance therapy was 50% (3/6) within the first 5 weeks. The three other patients of Foscarnet maintenance did not relapse after a follow-up period of 12 weeks. In contrast to ganciclovir, Foscarnet did not induce neutropenia but it produced kidney toxicity that led to reversible renal insufficiency in three cases. Thus, Foscarnet appears to be a useful alternative to ganciclovir, particularly when combined with bone marrow toxic drugs, such as zidovudine (azidothymidine).