A 4-month regimen of gatifloxacin with rifampin, isoniazid, and pyrazinamide is being evaluated for the treatment of tuberculosis in a phase 3 randomized controlled trial (OFLOTUB). A prior single-dose study found that gatifloxacin exposure increased by 14% in the combination. The aims of the study are to evaluate the initial and steady-state pharmacokinetics of gatifloxacin when daily doses are given to patients with newly diagnosed drug-sensitive pulmonary tuberculosis as part of a combination regimen and to evaluate the gatifloxacin dose with respect to the probability of attaining a pharmacokinetic/pharmacodynamic target. We describe the population pharmacokinetics of gatifloxacin from the first dose to a median of 28 days in 169 adults enrolled in the OFLOTUB trial in Benin, Guinea, Senegal, and South Africa. The probability of achieving a ratio of >125 for the area under the concentration time curve to infinity (AUC 0 -ؕ ) for the free fraction of gatifloxacin over the MIC (fAUC/MIC) was investigated using Monte Carlo simulations. The median AUC 0 -ؕ of 41.2 g · h/ml decreased on average by 14.3% (90% confidence interval [CI], ؊90.5% to ؉61.5%) following multiple 400-mg daily doses. At steady state, 90% of patients achieved an fAUC/MIC of >125 only when the MIC was <0.125 g/ml. We conclude that systemic exposure to gatifloxacin declines with repeated daily 400-mg doses when used together with rifampin, isoniazid, and pyrazinamide, thus compensating for any initial increase in gatifloxacin levels due to a drug interaction. (The OFLOTUB study has been registered at ClinicalTrials.gov under registration no. NCT00216385.) F luoroquinolones represent a promising class of drug for the treatment of tuberculosis. Gatifloxacin distributes widely throughout the body (1), achieving MICs for Mycobacterium tuberculosis observed in vitro of 0.031 to 0.5 g/ml (2, 3, 4), and demonstrates strong bactericidal activity in the mouse model (2, 5). Furthermore, gatifloxacin displays excellent early bactericidal activity (EBA), only slightly lower than that of isoniazid (6); replacing ethambutol with gatifloxacin in the standard first-line regimen resulted in accelerated killing of M. tuberculosis in the sputum of patients with pulmonary tuberculosis (7). A single-dose crossover study in healthy volunteers showed a reduction in the elimination rate of gatifloxacin resulting in a 14% increase in the area under the concentration time curve to infinity (AUC 0 -ϱ ) when it was given together with rifampin, isoniazid, and pyrazinamide (8). Reports of dysglycemia related to the use of gatifloxacin in elderly patients with renal insufficiency (9, 10, 11, 12) have raised concerns that pharmacokinetic interactions may lead to an increased risk of toxicity related to higher gatifloxacin exposure. On the other hand, in vitro and in vivo studies suggest a target ratio of Ն125 for the free drug area under the concentration versus time curve to MIC (fAUC/MIC) for maximal bactericidal effect and prevention of resistance to fluoroquinolones (13...