A controlled comparison of 3 short-course regimens was undertaken in patients with newly diagnosed, sputum-positive, pulmonary tuberculosis in South India. The regimens were: R3: rifampin plus streptomycin plus isoniazid plus pyrazinamide daily for 3 months; 5: the same as regimen R3 followed by streptomycin plus isoniazid plus pyrazinamide twice weekly for 2 months; Z5: the same as regimen R5 but without rifampin. The distributions of various pretreatment characteristics were similar in the 3 series. At the end of treatment, 6 patients (3 R3, 3 Z5) of 694 (228 R3, 230 R5, 236 Z5) with drug-sensitive organisms initially were classified as having an unfavorable response. By 24 months (21 months of follow-up for the R3 regimen and 19 months for the R5 and Z5 regimens), a bacteriologic relapse requiring treatment occurred in 20% of 200 R3, 4% of 187 R5, and 13% of 199 Z5 patients, the difference between the R3 and R5 series being highly significant (p = 0.00001). Considering patients with cultures initially resistant to isoniazid, 4 of 57 in the R3 and R5 series combined had an unfavorable response to treatment compared with 13 of 26 in the Z5 series (p less than 0.0001). Of the 4 patients with an unfavorable response in the R3 and R5 series combined, resistance to rifampin emerged in 2. Complaints of arthralgia were made by 45% of the R3 and R5 patients combined and 70% of the Z5 patients (p less than 0.00001). However, chemotherapy was modified in only 5 and 12%, respectively. Jaundice occurred in 7% of the R3 and R5 patients and 1% of the Z5 patients (p less than 0.00001).