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Background: The Foscarnet-Ganciclovir Cytomegalovirus Retinitis Trial compared the use of either ganciclovir or foscarnet for the initial treatment of cytomegalovirus retinitis in patients with the acquired immunodeficiency syndrome. We previously reported that patients treated with foscarnet lived longer but were more likely to have their treatment switched, the latter suggesting foscarnet may not have been as well tolerated as ganciclovir. This study compared the morbidity and toxic reactions reported during the trial., Methods: Two hundred thirty-four patients with the acquired immunodeficiency syndrome and previously untreated cytomegalovirus retinitis at 11 university centers were randomly assigned to receive intravenously either foscarnet (n = 107) or ganciclovir (n = 127). Medical histories, laboratory tests, and drug treatment histories during the first 6 months of treatment were analyzed., Results: Neutropenia was more common in patients assigned to ganciclovir than to foscarnet (34% vs 14%; P = .001). Patients assigned to foscarnet reported more infusion-related symptoms (58% vs 24%; P < .001) and, in male patients, more genitourinary symptoms (36% vs 16%; P > .001); they also experienced a trend toward more nephrotoxic effects (13% vs 6%; P = .082) and electrolyte abnormalities. The incidence of seizures was similar in both groups (foscarnet, 12%; ganciclovir, 9%; P = .511). Patients assigned to foscarnet were more likely to be switched to the alternative treatment (foscarnet to ganciclovir, 46%; ganciclovir to foscarnet, 11%; P < .001), and most of this excess was attributable to toxic reactions. In 88% of cases in which treatment was switched as a result of toxic reactions and in which follow-up data were available, the toxic reaction resolved after the switch. No permanent disability or death resulted from toxic reactions., Conclusions: Compared with ganciclovir, the use of foscarnet was more frequently limited by the occurrence of toxic reactions. However, these toxic reactions rarely had long-term sequelae. In light of the previously reported survival benefit seen in patients treated with foscarnet, these data support the use of foscarnet for the initial treatment of cytomegalovirus retinitis.