Although diffuse panbronchiolitis (DPB) has carried a poor prognosis, long-term low-dose administration of erythromycin (EM) is very effective. We administered EM at a daily dose of 400–600 mg to 19 DPB subjects for more than 2 months. Sixteen subjects were relieved from productive cough and dyspnea, and their chest X-ray pictures were improved. We performed a pharmacokinetic study of EM in 11 DPB subjects (8 re-sponders; 3 nonresponders) after the long-term low-dose administration. The maximal serum and sputum levels of EM were below the MICs of clinically pathogenic H. influenzae and P. aeruginosa which were often isolated from the sputum of DPB patients. No difference was observed in the absorption of EM between responded and nonresponders. The results suggested that DPB patients might respond favorably to EM due to mechanisms other than antibacterial activity. Individual variation in the absorption of EM was observed. As EM was effective at very low serum and sputum levels, it was suggested that even 200 mg/day of EM would be effective in DPB patients who had high serum and sputum EM levels and it was necessary to monitor the concentrations of EM in serum and sputum for the treatment of DPB to determine the appropriate dose of EM individual