Two 6-month regimens of isoniazid and rifampicin supplemented for the first 2 months by streptomycin and pyrazinamide or by ethambutol and pyrazinamide were compared with a 9-month regimen of isoniazid and rifampicin supplemented for the first 2 months by ethambutol. All 444 patients who completed chemotherapy had negative sputum cultures by the end of treatment. Of these, 393 have been followed for 24 months after the end of chemotherapy. Relapse has occurred in 1 of 125 SHRZ6 patients, 3 of 132 EHRZ6, and 2 of 136 EHR9 patients. These observations suggest that both 5-month regimens are as effective as the currently recommended 9-month regimen and have the advantages of being shorter, cheaper, and equally well tolerated.