In this double-blind, dose-escalating study, patients receive fozivudine tidoxil at one of 5 dosage levels for 4 weeks and are randomized with respect to once- or twice-daily administration (cohorts 2 vs. 3 and 4 vs. 5). Within each cohort, 10 patients are randomized to the study drug and 2 to the placebo. At least 9 of the 12 patients enrolled in Cohort 1 must complete the entire 4-week course before Cohorts 2 and 3 are enrolled. At least 18 of these 24 patients must complete 2 weeks of the 4-week course before Cohorts 4 and 5 are enrolled.