The prevalence of Mycobacterium tuberculosis (MTB) has increased worldwide, in part due to the HIV epidemic. Epidemiology data have demonstrated that HIV-infected individuals are more susceptible to MTB disease, which may lead to an acceleration in the progression of HIV disease. The purpose of this study was to determine whether MTB modulates HIV infection in vivo and to delineate the mechanisms involved by using in vitro model systems. Plasma viral load was measured in HIV-infected individuals before, during, and after the development of MTB disease; a 5- to 160-fold increase in viral replication was observed during the acute phase of MTB disease. In order to evaluate the mechanisms involved in this MTB-induced HIV replication, we used an in vitro system of primary PBMC and lymph node mononuclear cells isolated from HIV-infected individuals. The data demonstrated that MTB induced HIV replication in CD8+ T cell-depleted lymphocytes from HIV-infected individuals with a history of purified protein derivative (PPD) positivity but not in those who were PPD negative; this induction of HIV replication correlated with the level of cellular activation. In an in vitro acute HIV infection model, MTB increased HIV replication in PBMC from healthy donors with a history of PPD positivity, but not in PBMC from PPD-negative donors and this induction of viral replication also correlated with cellular activation. In conclusion, MTB increased HIV replication in vivo and in an in vitro model. This MTB-mediated viral production likely occurs through Ag-specific activation and infection of responding T cells.