We conducted a randomized, open-label, 10-day study that compared the antiretroviral activity of emtricitabine (FTC) 25, 100, and 200 mg once daily and lamivudine (3TC) 150 mg 2 times/day in 82 human immunodeficiency virus (HIV)-infected patients with virus loads >5000 and <100,000 copies/mL who were naive for 3TC and abacavir. All FTC doses demonstrated potent antiretroviral activity. Significantly greater virus suppression was seen at the 200 mg/day dose of FTC than with the lower FTC doses and/or 3TC (P=.02, P=.04, and P=.04, respectively). At the 200 mg/day dose, FTC produced a 1.7-log10 mean reduction in virus load. Trough FTC levels at the 200 mg/day dose exceeded the in vitro 90% inhibitory concentration dose for FTC by 5-fold. The long plasma half-life and the superior antiviral activity versus 3TC of the 200 mg/day FTC dose confirmed the results of other studies and led to the selection of this dose for subsequent therapeutic trials.