The effects of pharmacists' interventions on patient outcomes in an HIV primary care clinic were studied. All study participants were referred to a pharmacist-managed drug optimization clinic (DOC) in a county-based HIV primary care clinic between November 1, 2003, and September 30, 2004. Patients were eligible for study participation if they were 18 years of age or older and gave informed consent to participate. Pharmacists' interventions were categorized as follows: patient education, addition of a medication, dosage adjustment, discontinuation of a medication, and interpretation of viral-resistance tests. Changes in baseline CD4+ T-lymphocyte counts and viral load were also measured over the study period. Toxicities related to highly active antiretroviral therapy were recorded and graded from 0 to 4, with 0 indicating no toxicity and 4 indicating severe toxicity. Study participants used a standardized survey to measure their own health-related quality of life. Changes in CD4+ lymphocyte counts and viral load were analyzed using Student's t test and analysis of variance. Toxicity grades were analyzed using the Wilcoxon signed-rank test. A total of 34 patients completed the study. Pharmacists made a total of 253 interventions, most of which were categorized as patient education. The mean CD4+ lymphocyte count increased from baseline levels by 54 +/- 78 cells/mm3 over the study period (p < 0.0002). The mean +/- S.D. reduction in circulating viral load over the study period was 1.02 log10 copies/mL ( p < 0.004). HIV-infected patients who were managed by pharmacists in a DOC demonstrated significant improvement from baseline in their CD4+ lymphocyte counts, viral loads, and drug-related toxicities.