Multi-drug-resistant tuberculosis (MDR-TB) affects nearly 600,000 persons each year around the world. This type of tuberculosis is very difficult to treat, and many patients die from it. Drugs of the fluoroquinolone class are very important for treating MDR-TB, but the best dose of one of the most effective fluoroquinolones, levofloxacin, is not known. This application proposes a study to determine the best dose of levofloxacin to use in treating MDR-TB. 120 patients will receive their usual treatment, plus levofloxacin at one of four doses. The study will be performed in Peru and in South Africa, where MDR-TB is common.