The emergence of strains of Mycobacterium tuberculosis that are resistant to antimycobacterial agents is a worldwide problem whose global magnitude is not well described. We reviewed and tabulated 63 surveys of resistance to antituberculous drugs that were performed between 1985 and 1994. Rates of primary resistance to isoniazid, administered as a single agent, ranged from 0 to 16.9% (median rate, 4.1%); to streptomycin, 0.1%-23.5% (median, 3.5%); to rifampin, 0-3.0% (median, 0.2%); and to ethambutol, 0-4.2% (median, 0.1%). The rates of acquired resistance to these agents, which were higher than those of primary resistance, were as follows: isoniazid, 4.0%-53.7% (median rate, 10.6%); streptomycin, 0-19.4% (median, 4.9%); rifampin 0-14.5% (median, 2.4%); and ethambutol, 0-13.7% (median, 1.8%). The highest rates of multidrug-resistant tuberculosis have been reported in Nepal (48.0%), Gujarat, India (33.8%), New York City, (30.1%), Bolivia (15.3%), and Korea (14.5%). The WHO (World Health Organization) and the IUATLD (International Union Against Tuberculosis and Lung Disease) have established a global project of drug resistance surveillance that is based on standard epidemiological methods and quality control through an extensive network of reference laboratories. Accurate drug resistance surveillance data can be used to assess and improve national tuberculosis programs.