In the treatment of diffuse panbronchiolitis, azithromycin (AZM), a new macrolide antibiotic with 15-membered lactone ring, was studied for its efficacy and safety. AZM, 250 mg, was intermittently administered to a total of 60 patients twice a weeks, for 3 months as a rule, and its efficacy was clinically evaluated in 52 patients and the safety in 55. The rate of efficacy was 84.6% (44/52). Clinical findings 12 weeks after the start of administration showed a decrease in sputum volume in 30 of 46 patients and amelioration of dyspnea on exertion in 23 of 46 patients, and no worsening of symptoms was observed in the patients. Vital capacity (4/22), FEV1.0 (6/21), cold agglutination reaction (22/28), and CRP (16/36) were also improved. The rate of eradication of organisms isolated from the sputum except for indigenous organisms was 39.5% (15/38); 4 of the 22 strains of Pseudomonas aeruginosa were eradicated. Adverse reactions were observed in 4 of the 55 patients (7.3%), 1 patient each with rash, itching, diarrhea, and a gastric symptom (heavy feeling in the stomach). 4 of the 54 patients (7.4%) exhibited abnormal changes in clinical laboratory test values values. These were an increase in eosinophil count in 2, elevation of GOT in 1, and elevation of Al-P in 1. These adverse reactions and abnormal changes in laboratory tests were mild or moderate. Therefore, long-term intermittent administration of AZM, twice a week, is expected to have the same effect in the treatment of diffuse panbronchiolitis as long-term small-dose administration of 14-membered macrolides such as erythromycin and clarithromycin, whose effects have already been established.