OBJECTIVE: The effects on T-lymphocyte populations of two interferon-alfa-2a (IFN) regimens associated with ribavirin were evaluated in 36 HCV-HIV co-infected patients with chronic hepatitis C, T-CD4 cell count > 250 cells/ micro L and a plasma viral load of < 10 000 HIV RNA copies/mL. METHODS: Patients were given IFN for 48 weeks. Group A (18 patients) received 6 mega units (MU) subcutaneously three times a week for 24 weeks, then 3 MU three times a week for the last 24 weeks. Group B (18 patients) received 9 MU daily for 2 weeks, 3 MU daily for 22 weeks, then 3 MU three times a week for the last 24 weeks. Serum HCV RNA was evaluated at weeks 12 and 72. Ribavirin was added at week 16 for virologic nonresponders at week 12. CD3, CD3 CD4, CD3 CD8, CD3 CD4 human leucocyte antigen (HLA)-DR and CD3 CD8 HLA-DR lymphocyte subsets were evaluated before, during and after treatment by cytofluorometry. Controls were healthy and HCV mono-infected patients. RESULTS: CD3 CD4 and CD3 CD8 T-cells counts were both impaired during anti-HCV therapy, but returned to baseline value after treatment completion. Lymphopenia concerned mainly CD8 T-cells, the percentage of which decreased, whereas that of CD4 increased. Three patients displayed reversible CD4 lymphopenia < 200 cells/ micro L. HIV infection at inclusion was responsible for higher CD3 CD8 HLA-DR T-cell percentages in co-infected patients than in healthy and HCV mono-infected subjects. T-cell sequestration in lymphoid tissues and enhanced apoptosis may account for lymphopenia. CONCLUSION: High-dosed IFN anti-HCV therapy induced only moderate and transient CD4 lymphopenia in HIV co-infected patients.