Three hundred and twenty-nine patients with isoniazid-resistant cultures, 66% with radiographically far advanced disease and 86% with cavities, have been treated with rifampicin and ethambutol and followed-up for 2 years after the end of treatment. The drugs were given daily for 12 weeks (600 mg rifampicin and 25 mg/kg ethambutol), thereafter once- or twice-weekly (600 or 1200 mg rifampicin and 50 mg/kg ethambutol) for a total of 12, 18 or 24 months (in the 600 mg group for 12 months only). With both 600 and 1200 mg rifampicin dosage the bacteriological results at the end of a year were similar (5% bacteriologically unfavourable in each group). Prolonging treatment to 18 or 24 months with the 1200 mg rifampicin dose had no effect on the bacteriological results. During the 2 years follow-up period after treatment stopped 6 patients had a bacteriological relapse. Of the 74 with a favourable status after 1 year in the 600 mg rifampicin group 5 (6.7%) relapsed, but only 1 (0.6%) of 168 in the 1200 mg groups treated for 12, 18 or 24 months (the duration of treatment did not appear to be related to relapse). Side-effects were reported more frequently with once-weekly dosage. They were more frequent with 1200 mg rifampicin than with 600 mg and with the former dose more frequent in the groups treated for longer than 1 year. With 12 months treatment with 600 mg rifampicin only 1% of patients had to have the regimen changed; with 1200 mg it was 9% and with this dose for 24 months 20%. With the lower dosage of rifampicin there were fewer failures due to toxicity but more failures due to relapse. Of 82 patients in the 600 mg regimen there were 12% unfavourable results (4 bacteriological failures, 5 relapses and 1 change of treatment for toxicity). Of 78 patients in the 1200 mg regimen there were 13% unfavourable results (3 bacteriological failures, no relapses and 7 changes of treatment for toxicity).