Low dose and long-term erythromycin (EM) therapy showed obvious improvement of symptoms and prognosis in patients with DPB. However, mechanisms involved in these clinical effects of EM therapy remain controversial. In this study, the clinical effects of EM therapy on respiratory function, bacterial flora in the lower airway and serum levels of soluble IL-2 receptor (sIL-2R) as an indicator of T-cell activation were examined. All parameters of respiratory functions in patients with DPB including % VC, % FEV 1.0, % V50, % V25 and V50/V25 showed marked improvements within 6 months of EM therapy. Continuous administration of EM for 6 to 24 months produced minimal changes in these respiratory functions though decreases in some parameters were observed in patients with advanced clinical stage and in eldely patients. The increase of % FEV 1.0 exceeded those of % V50 and % V25 after EM therapy. This indicates severe pathological changes in small airways or variable effects of EM therapy. Bacterial species and sensitivities to antibiotics showed no apparent changes after long-term EM therapy. The serum levels of sIL-2R in patients with DPB were higher than those in normal controls. With EM therapy, serum levels of sIL-2R decreased rapidly within 6 months, but these levels remained higher than those in normal controls, even after long-term EM therapy.