Whether the development of active tuberculosis in people with previous tuberculous infection represents an episode of endogenous reactivation or exogenous reinfection has been debated for decades. Articles proposing the unitary concept of pathogenesis of tuberculosis in the 1960s initiated a period in which reinfection was considered to be an uncommon cause of tuberculosis. To review evidence demonstrating the occurrence of tuberculosis due to exogenous reinfection, we did a literature search covering publications from 1966 until the present, and found that there was substantial evidence--both experimental and epidemiological--supporting the role of exogenous reinfection in tuberculosis. However, only models based on estimates of the annual risk of infection and the incidence of tuberculosis provided a quantitative estimate of the relative contribution of exogenous reinfection to the burden of tuberculosis. Better estimates of the contribution of exogenous reinfection to new cases of tuberculosis may need to be considered in tuberculosis control strategies.