Fluoroquinolones (FQ) are an essential part of regimens for the treatment of MDR-TB; substantially better outcomes have consistently been seen in patients with MDR-TB who are treated with FQ, and newer FQ (levofloxacin, gatifloxacin and moxifloxacin) are the most potent antituberculosis agents available for MDR-TB treatment. However, gatifloxacin has been taken off the market because of dysglycemic reactions and moxifloxacin produces marked QT prolongation, increasing risk of fatal arrhythmia. In contrast, QT studies of levofloxacin have found minimal prolongation at doses up to 20mg/kg. Levofloxacin is currently given for TB at doses of 11-14 mg/kg/day and has been well tolerated at doses up to 20 mg/kg. Although the efficacy of levofloxacin increases as exposure increases both in animal studies of TB and in human studies of gram-negative bacteria, its efficacy at higher doses against TB in humans has not been studied. Thus, determination of the most efficacious and well-tolerated dose of levofloxacin is an important research priority. In this Phase 2 study, we will determine the levofloxacin dose and exposure that achieve the greatest reduction in mycobacterial burden with acceptable tolerability by studying 120 adults with smear- and culture-positive pulmonary MDR-TB at sites in Peru and South Africa. Levofloxacin will be administered with an optimized background regimen (OBR) to address the following Specific