The choice of an appropriate chemotherapeutic regimen for the treatment of tuberculosis is more difficult if the patient has been treated for tuberculosis in the past. This study was undertaken to determine drug-resistance rates among patients previously treated with isoniazid (INH), streptomycin (SM), and/or paraaminosalicylic acid (PAS). The study population consisted of 4,017 patients for whom the length and type of previous therapy was known. Forty-one per cent of these patients were found to be excreting organisms resistant to 1 or more of the following 3 drugs: INH, SM, and PAS. Resistance to INH was encountered most (36.8%), followed by resistance to SM (19.2%), and resistance to PAS (17.2%). Resistance rates were considerably higher among the 1,168 patients who had previously received monotherapy (60%) than among those who had never received single-drug therapy (33%). In general, the percentage of patients excreting resistant organisms increased with increasing duration of the previous therapy. The implications of these findings for the design of retreatment regimens are discussed.