CMV retinitis is a common opportunistic infection in patients with AIDS. Ganciclovir is currently the only drug approved for treatment of CMV retinitis in immunocompromised patients. Ganciclovir suppresses CMV infections, and relapse occurs in virtually all AIDS patients when ganciclovir is discontinued. Because of their similar hematologic (blood) toxicities, the simultaneous use of ganciclovir and zidovudine (AZT) is not recommended. More recently the drug foscarnet has become available for investigational use. Studies so far indicate that remission of CMV retinitis occurs in 36 to 77 percent of patients, and that relapse occurs in virtually all patients when the drug is discontinued. The relative effectiveness of foscarnet compared with ganciclovir for the immediate control of CMV infections is unknown. Further, the long-term effects of foscarnet or ganciclovir on CMV retinitis, survival, and morbidity are unknown. There is also no definitive information on the relative effectiveness and safety of deferred versus immediate treatment for CMV retinitis confined to zones 2 and 3.