We have investigated the efficacy of a clarithromycin-containing four-drug regimen for Mycobacterium avium complex (MAC) pulmonary disease in 46 patients without acquired immunodeficiency syndrome (AIDS). The patients were 14 males and 32 females with a mean age of 60.9 +/- 11.5 yr. Patients received 10 mg/kg/d of clarithromycin plus ethambutol, rifampin, and initial kanamycin and subsequent quinolone for 24 mo. Seven patients (15.2%) were dropped in the first 6 mo. Among 39 patients who received more than 6 mo of therapy, 28 patients (71.8%) converted their sputa to negative: 26 of 31 patients (83.9%) infected with clarithromycin-susceptible strains and two of eight patients (25.0%) with resistant or intermediate strains. The timing of sputum conversion was 3.6 +/- 1.9 mo, with a range of 2 to 9 mo. The conversion rate was significantly lower in patients who were infected with clarithromycin-resistant or intermediate strains, who had had prior therapy (55.0% versus 89.5%), or who were acid-fast bacilli (AFB) smear-positive at entry (60.7% versus 100%). The age and sex of patients, the species of pathogen (M. avium or M. intracellulare), type and extent of the disease, and the use of kanamycin did not significantly affect the conversion rate. Although the regimen was efficacious for newly treated patients, frequent adverse reactions and a low conversion rate of sputum in retreated patients are problems that remain to be solved.