A longitudinal study of human immunodeficiency virus (HIV)-infected individuals followed-up in 13 centres was performed to assess the influence of hepatitis C virus (HCV) on the clinical and immunological evolution of HIV-infected patients. Eight-hundred and twelve HIV-infected patients with known HIV acquisition date, 89 co-infected with HCV, were included in the cohort. Clinical progression was defined as: 30% decrease of Karnofsky's index; and/or 20% body weight loss; and/or acquired immune deficiency syndrome (AIDS)-defining illness; and/or death (except by accident, suicide, or overdose). Immunological progression was defined as a decrease of initial CD4 count to below 200 mm(-3). If immunological progression was not statistically different between groups (P=0.25), clinical progression was significantly faster in HCV-HIV co-infected patients in univariate (P=0.02) and multivariable survival analysis (hazard ratio=1.63, P=0.03). This argues for active management of hepatitis C chronic infection among HCV-HIV co-infected patients.