Lamivudine is a novel cytosine nucleoside analog, reverse transcriptase inhibitor that has shown activity against human immunodeficiency virus (HIV) types 1 and 2 and hepatitis B virus in vitro. This study was conducted to compare the absolute bioavailability, pharmacokinetics, and absorption characteristics of oral solution, 100-mg capsule, and 100-mg tablet formulations of lamivudine with those of intravenous lamivudine. Twelve patients with HIV were enrolled in a single-center, randomized, open-label, four-way cross-over study. Treatment arms consisted of 100 mg intravenous lamivudine (administered over 1 hour), 100 mg oral lamivudine (1 mg/mL), a 100-mg capsule, and a 100-mg tablet, each followed by a 3- to 14-day washout period. Serial blood samples over 24 hours were obtained after each dose administration. Serum concentration data were analyzed to determine pharmacokinetic parameter estimates including area under the curve (AUC), terminal half-life (t1/2), mean residence time (MRT) for each formulation, systemic clearance, oral clearance, and apparent volume of distribution (Vd). Absolute bioavailability and in vivo mean absorption time (MAT) and mean dissolution time (MDT) were calculated for the oral formulations. Deconvolution techniques were used to calculate the input rate for the oral solution, capsule, and tablet. The two one-sided t test was used to determine bioequivalency among oral formulations with respect to logarithmic transformed estimates of AUC and maximum peak concentration (Cmax). Mean (CV) systemic clearance and Vdss after intravenous administration of lamivudine were 22.6 L/h (15%) and 99 L (28%), respectively; mean t1/2 ranged from 8.41 to 9.11 hours for all formulations; and MRT ranged from 4.42 to 5.77 hours for all formulations. Mean absolute bioavailability ranged from 86% to 88% for the oral solution, capsule, and tablet. All oral formulations were considered bioequivalent for AUC and Cmax. The MAT was 1.32 hour for the oral solution, and MDT was 0.03 and -0.11 hours for the capsule and the oral solution, respectively. The oral formulations of lamivudine examined in this study demonstrated acceptable bioavailability for oral administration. The solid oral formulations (capsule and tablet) show rapid dissolution properties with an absorption rate similar to or exceeding those observed with the oral solution. This suggests that dissolution is not an important factor for the rate of absorption of lamivudine. The use of deconvolution techniques using PCDCON provides valuable insight into the absorption characteristics of lamivudine.