Tuberculosis is the most common opportunistic infection in HIV-infected people living in developing countries and is believed to accelerate the progression of HIV disease. This effect may be mediated by increased immune activation. We measured levels of CD4 and CD8 T-lymphocyte activation and proliferation in control subjects, patients with HIV alone, TB alone and patients with HIV and TB co-infection. Our results indicate that TB (in the absence of HIV) increases T-lymphocyte proliferation but its effects are modest in comparison with the stimulation induced by HIV infection alone.