While the detrimental consequences of opportunistic tuberculosis (TB) in the course and outcome of HIV-1 infection are well studied, little information about the impact of the mycobacterial infection on the phenotype of T lymphocytes is available. In this study we analyzed by cytofluorimetry the peripheral blood T cell phenotype of 13 patients with AIDS, 23 HIV-1 negative patients with active pulmonary TB, nine HIV-1/Mycobacterium tuberculosis coinfected individuals, and 21 age-and sexmatched healthy controls. CD41 T cells were equally depleted in AIDS and coinfection (Po0.001). The findings suggest a rescuing effect of the added mycobacterial infection. CD3 T cell loss was not observed in coinfection, whereas it was severe in AIDS (Po0.001). Similar (albeit less striking) effects were observed with other markers (CD45RA, CD45RO, and CD27) that were diminished in CD41 T cells of AIDS patients. Apparent detrimental effects of the added mycobacterial infection were the increased expression of the proapoptotic molecule CD95 on CD41 T cells, and decreased expression of the major costimulatory molecule CD28 on CD81 T cells. In this work we show that M. tuberculosis infection modifies the T cell phenotype of the HIV-1 infected individual.