3Current affiliation: World Health Organization, Geneva, Switzerland untreatable forms of drug-resistant TB that had not been described previously in different parts of the world, we characterized and quantified the frequency of secondline–drug resistance in several geographic regions. We sought to determine the extent to which highly resistant Mycobacterium tuberculosis strains have been identified by the international laboratories that participate in the Network of Supranational Reference Laboratories (SRLs). The SRL Network consists of 25 highly proficient TB laboratories on 6 continents. These laboratories collaborate with national reference laboratories to strengthen culture and drug-susceptibility testing capacity and to provide quality control for the WHO/International Union Against Tuberculosis and Lung Diseases Global Project on Anti-TB Drug Resistance (14).