Molecular and conventional epidemiologic techniques were used to study the mechanisms and risk factors for tuberculosis transmission in a rural area with high prevalence in south India. Restriction fragment length polymorphism analysis with IS6110 and direct repeat probes was performed with 378 Mycobacterium tuberculosis isolates from patients. Forty-one percent of M. tuberculosis isolates harbored a single copy of IS6110. Of 378 patients, 236 had distinct strains; 142 (38%) shared a strain with other patients, indicating recent infection. Older patients, those detected by a house-to-house community survey, and those hospitalized in a sanatorium were more likely to have had a recent infection. These findings suggest that the majority of the tuberculosis cases in south India were due to reactivation; therefore, efforts to control tuberculosis should be sustained.