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BCG(Bacille Calmette-Guerion) vaccination to control tuberculosis is widly practiced around the world, but conflicting results from several large-scale trials and case-control studies have cast doubt on its effectiveness. Despite the disappointing results of some studies, however, expert committees of the world Health Organization continue to recommend the mass use of BCG vaccination in developing countries, and stress the need for further research. This paper reviews some major conflicting results on the vaccine efficacy of BCG against tuberculosis and discusses the causes and effects of the variations. Major controversial explanations on BCG’s varying efficacy are infection status of certain non-tuberculosis mycobacteria, differences between BCG’s, differences in natural history of infection and disease, variations in host genetics or nutrition, and methodological differences in the trials.