In a study in Singapore, Chinese, Malay and Indian patients with pulmonary tuberculosis received 2 months of daily treatment with streptomycin, isoniazid, rifampicin, and pyrazinamide followed by daily isoniazid, and rifampicin either with pyrazinamide (SHRZ/HRZ) or without it (SHRZ/HR), allocated at random. Both regimens were given for either 6 or 4 months by random allocation. All 330 patients with drug-sensitive tubercle bacilli pretreatment had a favourable bacteriological response during chemotherapy. After chemotherapy none of 78 SHRZ/HRZ patients and only 2 of 80 SHRZ/HR patients treated for 6 months relapsed bacteriologically, but 9 (11%) of 79 SHRZ/HRZ and 6 (8%) of 77 SHRZ/HR patients treated for 4 months relapsed. Of 33 patients with bacilli resistant to isoniazid, streptomycin, or both drugs pretreatment, only 1 had an unfavourable response during chemotherapy; none of 9 patients treated for 6 months and 2 of 22 treated for 4 months relapsed bacteriologically after stopping chemotherapy.