Diarrhea in patients with acquired immunodeficiency syndrome (AIDS) may cause malabsorption of medications and failure of antiretroviral therapy (ART). We prospectively evaluated human immunodeficiency virus-1 (HIV-1)-infected patients with and without chronic diarrhea initiating ART in Haiti. We report mean plasma antiretroviral concentrations at 2 and 4 weeks. We measured plasma HIV-1 RNA levels at four points. Fifty-two HIV-1-infected patients (26 matched pairs) were enrolled. No differences in antiretroviral concentrations were detected. At week 24, 18/25 (72%) cases and 16/24 (68%) controls had undetectable plasma HIV-1 RNA levels (P = 0.69). Patients with plasma HIV-1 RNA levels > 50 copies/mL at week 24 had lower early efavirenz concentrations than patients with undetectable HIV-1 RNA (2,621 ng/mL versus 5,278 ng/mL; P = 0.02). Diarrhea at ART initiation does not influence plasma concentrations of the medications evaluated. Virologic outcome at Week 24 does correlate with efavirenz concentrations early in therapy but not with the presence of chronic diarrhea.