 On a global scale, tuberculosis TB is the single most important infectious cause of morbidity and mortality. The World Health Organization has estimated that one-third of the entire worlds population carries latent TB infection. A key TB control strategy is therapy of latent TB infection LTBI. The current standard regimen is 9 months of Isoniazid 9INH. This regimen has excellent efficacy if taken regularly, but its effectiveness is substantially reduced by poor compliance. Serious side effects, such as hepato-toxicity can occur. Three shorter alternatives have been recommended: 6 months INH 6INH, 2 months Rifampin - Pyrazinamide 2RIF-PZA and 4 months Rifampin 4RIF. The regimen of 6INH is less efficacious than 9INH, while 2RIF-PZA has been largely abandoned because of serious toxicity. Based on some evidence in treatment of LTBI, and extrapolating from extensive experience with treatment of active TB, it is believed that 4RIF has similar efficacy as 9INH. Therefore, the investigators are initiating the first multi-site international randomized trial that will compare the effectiveness of 4RIF and 9INH in preventing active tuberculosis. 