A total of 577 Danish patients with tuberculosis were observed for a period of 5 years. A primary phase of treatment with 300 mg Isoniazid (INH), 450 mg Rifampicin (RMP) and 1200 mg Ethambutol (EMB) daily for 3 months was followed by administration of either INH+RMP or INH+EMB for 12 or 18 months after conversion. During the initial period the number of bacteria decreased rapidly, even in patients with the most severe tuberculosis, and all patients became culture negative. There was no significant difference in efficacy of RMP and EMB in the secondary phase. One of the 577 patients again became positive during the follow-up period, but there were no bona fide cases of relapse among patients who completed the treatment.