Two 6-month regimens of isoniazid and rifampicin supplemented for the first 2 months by streptomycin and pyrazinamide (SHRZ6 regimen), or by ethambutol and pyrazinamide (EHRZ6 regimen), were compared with a 9-month regimen of isoniazid and rifampicin supplemented for the first 2 months by ethambutol (EHR9 regimen). All 444 patients who completed chemotherapy had negative sputum cultures by the end of treatment. Of these, 373 have been followed for a minimum duration of 36 months after the end of chemotherapy. Relapses have occurred in two of 119 SHRZ6 patients, four of 127 EHRZ6 patients and two of 127 EHR9 patients. These results demonstrate that the two 6-month regimens are as effective as the currently recommended 9-month regimen. They are equally well tolerated and have the advantages of being shorter and cheaper. The Research Committee of the British Thoracic Society now recommends the use of either of the two 6-month regimens as an alternative to the currently recommended 9-month regimen.