We report the first randomized prospective comparative study of long-term maintenance ganciclovir (9-[2-hydroxy-1-(hydroxymethyl)ethoxymethyl]guanine, BW759U, DHPG) therapy for cytomegalovirus retinitis in patients with the acquired immunodeficiency syndrome (AIDS). Eleven retinitis patients who received a 10-day course of ganciclovir induction therapy and then were randomized to receive either immediate daily ganciclovir maintenance therapy or deferred maintenance (eight deferred maintenance, three immediate maintenance) were evaluated for drug efficacy. Median time to retinitis progression was 42 days for the immediate maintenance group compared with 16 days for the deferred maintenance group, (P = 0.07). After crossing over to maintenance therapy, patients in the deferred group had a median time to retinitis progression of 58 days compared to 16 days while not on maintenance therapy (P = 0.13). Only 9% of cultures obtained while patients received maintenance therapy were positive for cytomegalovirus, vs 40% of those obtained off maintenance (P less than 0.001). We can state then that maintenance therapy with ganciclovir delays, but does not halt, progression of cytomegalovirus retinitis and suppresses, but does not eradicate, cytomegalovirus shedding in patients with AIDS.