Superoxide dismutase (SOD) is a ubiquitous metalloenzyme in aerobic organisms that catalyzes the conversion of superoxide anion to hydrogen peroxide. Mycobacterium tuberculosis is unusual in that it secretes large quantities of iron-cofactored SOD. To determine the role of SOD in pathogenesis, we constructed mutants of M. tuberculosis H37Rv with reduced SOD production. Compared with controls, SOD-diminished isolates were more susceptible to killing by hydrogen peroxide. The isolates were markedly attenuated, exhibiting nearly 100,000-fold fewer bacilli than virulent control strains in the lungs and spleens of C57BL/6 mice 4 wk after intravenous inoculation. In the lung, SOD-attenuated M. tuberculosis induced robust interstitial mononuclear cell infiltration within 24 h and many cells were apoptotic by TUNEL staining, whereas virulent H37Rv exhibited minimal early inflammatory response and only rare interstitial mononuclear cell apoptosis. During prolonged infections, C57BL/6 mice tolerated SOD-attenuated M. tuberculosis better than BCG, exhibiting 68% greater weight gain, quicker eradication of bacilli from the spleen, and less alveolar lung infiltration. These results establish the importance of SOD in the pathogenesis of tuberculosis. Its effect appears to be mediated in part by inhibiting innate host immune responses, including early mononuclear cell infiltration of infected tissues and apoptosis.