Rationale: Although observational studies suggest that clofaziminecontaining regimens are highly active against drug-resistant tuberculosis, the contribution of clofazimine for the treatment of this disease has never been systematically evaluated. Objectives: Our goal was to directly compare the activity of a standard second-line drug regimen with or without the addition of clofazimine in a mouse model of multidrug-resistant tuberculosis. Our comparative outcomes included time to culture conversion in the mouse lungs and the percentage of relapses after treatment cessation. Methods: Mice were aerosol-infected with an isoniazid-resistant (as a surrogate of multidrug-resistant) strain of Mycobacterium tuberculosis. Treatment, which was administered for 5 to 9 months, was initiated 2 weeks after infection and comprised the following second-line regimen: daily (5 d/wk) moxifloxacin, ethambutol, and pyrazinamide, supplemented with amikacin during the first 2 months. One-half of the mice also received daily clofazimine. The decline in lung bacterial load was assessed monthly using charcoal-containing agar to reduce clofazimine carryover. Relapse was assessed 6 months after treatment cessation. Measurements and Main Results: After 2 months, the bacillary load in lungs was reduced from 9.74 log 10 at baseline to 3.61 and 4.68 in mice treated with or without clofazimine, respectively (P , 0.001). Mice treated with clofazimine were culture-negative after 5 months, whereas all mice treated without clofazimine remained heavily culturepositive for the entire 9 months of the study. The relapse rate was 7% among mice treated with clofazimine for 8 to 9 months. Conclusions: The clofazimine contribution was substantial in these experimental conditions. Keywords: tuberculosis; multidrug-resistant tuberculosis; clofazimine; inbred BALB/c mice Clofazimine, also known as B.663 or Lamprene, is a fat-soluble riminophenazine dye developed in the 1950s by Vincent Barry and colleagues (1, 2) as a drug for the treatment of tuberculosis. The drug was shown to be even more bactericidal than isoniazid when tested in a mouse model of tuberculosis chemotherapy (2). For reasons that are not well documented, clofazimine was not advanced for the treatment of human tuberculosis but was used for the treatment of leprosy and later, unsuccessfully, for the treatment of Mycobacterium avium complex infection in patients with AIDS (3, 4). The unfortunate development of multidrug-and extensively drug-resistant tuberculosis has kindled a worldwide push for the development of new therapy options for this disease, including a renewed interest in clofazimine (5). Although several groups have characterized clofazimine activity for in vitro and animal models of tuberculosis (6), a seminal publication by van Deun and colleagues in 2010 reported for the first time that a 9-month drug regimen including gatifloxacin, ethambutol, pyrazinamide, and clofazimine throughout plus a 4-month initial supplement of kanamycin, prothionamide, and a high dose of isoniazid achi...