To find oral doses of FIAC (a pyrimidine nucleoside analog) that are effective in treating cytomegalovirus (CMV) viremia in HIV-infected immunocompromised patients; to determine tolerance and safety of FIAC in this patient population; and to determine pharmacokinetics following multiple doses of FIAC. (An example of another nucleoside analog effective against retroviruses such as HIV is zidovudine (AZT).) CMV infection is a medically significant opportunistic disease in patients with HIV-related infection. The purine nucleoside ganciclovir has been used to treat AIDS patients with CMV disease. Although ganciclovir is useful in treating CMV disease, such treatment is frequently complicated by hematologic (blood) toxicity. Also, treatment is difficult because it requires daily intravenous dosing. Test tube studies show that FIAC and its primary breakdown product FIAU are highly and specifically active against several viruses including CMV. A single-dose, pharmacokinetic (blood level) study showed that FIAC, when taken orally, is readily absorbed into the bloodstream, and most of it is converted to FIAU.