Thalidomide is used to treat human immunodeficiency virus (HIV)-associated conditions, including aphthous ulcers and wasting syndrome. The safety, tolerability, and pharmacokinetics of a formulation of thalidomide with improved bioavailability in HIV-infected persons was examined in a placebo-controlled, dose-escalating phase 1 study. Subjects with CD4 cell counts of 200-500 cells/mm(3) were enrolled and randomized 3:1 in groups of 12 to receive 50, 100, or 150 mg of thalidomide or matching placebo. Two subjects who received 150 mg of drug and 2 subjects assigned placebo experienced dose-limiting toxicity. Concentrations of thalidomide in the blood increased with escalating dose, but the time to maximum concentration and clearance did not differ across dose cohorts. Previous suggestions of autoinduction of drug metabolism were not confirmed by this study. At the doses studied, thalidomide was tolerated well and had linear pharmacokinetics.