OBJECTIVE: Few data exist on the efficacy of antiretroviral therapy in individuals infected with HIV in the Caribbean. We evaluated the virological and immunological responses of HIV-infected adults starting highly active antiretroviral therapy (HAART). DESIGN: This was a prospective observational cohort study. METHODS: A total of 158 antiretroviral-naive patients who initiated HAART between January 2002 and March 2003, and completed at least 6 months of treatment and follow up, were included in the analysis. The response to therapy was assessed by changes in CD4 cell counts and viral loads from baseline. The mean increase in CD4 cell count, the rate of virological success (a viral load of <50 HIV-1 RNA copies/mL) and the rate of immunological success (an increase in CD4 cell count of > or =50 cells/microL over the baseline value) after commencing HAART were measured. RESULTS: In total, 82% of patients (123 of 150) achieved viral loads of <50 copies/mL after 6 months of therapy. Viral success rate after 6 months of HAART was similar irrespective of gender, pre-HAART CD4 cell count and pre-HAART viral load. However, patients older than 40 years were significantly more likely to achieve virological success than those younger than 40 years. At 6 months after starting HAART, 79.5% of patients were estimated to have achieved immunological success and 17.9% had an increase in CD4 cell count of > or =200 cells/microL over the baseline value. The median increase in CD4 cell count for the 156 patients who had CD4 cell counts at baseline and at 6 months of therapy was 122 cells/microL. CONCLUSION: In this cohort of antiretroviral-naive HIV-infected adults, there was a high rate of virological and immunological success after 6 months of HAART, irrespective of the pre-HAART viral load and CD4 cell count.