Four daily short-course chemotherapy regimens (three 6-month and one 8-month) for pulmonary tuberculosis have been compared. All four had the same initial 2-month intensive phase of streptomycin, isoniazid, rifampicin and pyrazinamide (SHRZ). The continuation phase of the 6-month regimens was: 1) isoniazid and rifampicin (4HR), or 2) isoniazid and pyrazinamide (4HZ), or 3) isoniazid alone (4H) and of the 8-month regimen 4) isoniazid alone (6H). All patients have been followed up for 12 months after stopping chemotherapy. In patients with fully sensitive strains pretreatment the 6-month regimen with rifampicin throughout (4 HR) was highly effective with a bacteriological relapse rate of 2% of 167 patients, significantly better (P less than 0.01) than the rate of 9% of 158 patients in the 6-month regimen with isoniazid alone in the continuation phase (4H). The 8-month regimen (6H) was also highly effective, with a relapse rate of 3% of 119 patients but was not significantly better (p greater than 0.1) than the 6-month isoniazid regimen (4H). The regimen with pyrazinamide throughout (4HZ) had a relapse rate of 4% of 165 patients, not significantly different from any of the other regimens. An important finding in the two regimens with isoniazid alone in the continuation phase (4H and 6H) was that 1 of the 2 failures during chemotherapy and 17 of the 18 bacteriological relapses after stopping occurred with strains still sensitive to isoniazid. In patients with strains resistant to isoniazid pretreatment, 8 of 19 patients on the 4H or 6H regimens had an unfavourable response during chemotherapy compared with none of 26 on the HR or HZ regimens (P less than 0.005). Of the 851 patients who started treatment, 24 developed possible adverse reactions but only 6 required modification of their chemotherapy.