Diffuse panbronchiolitis (DPB) was first distinguished from chronic obstructive pulmonary diseases in the early 1960s and recorded as a new clinicopathologic entity. This disease affects East Asians for the major part, and is characterized by chronic sinobronchial infection together with diffuse bilateral micronodular pulmonary lesions consisting of inflammatory cells. The prognosis at an advanced stage would often be bleak, where superinfection with Pseudomonas aeruginosa had occurred. Beginning with an initial success of erythromycin therapy, considerable improvement has been achieved in the prognosis of this disease. Simple bactericidal activity of macrolides is not a determinant factor for the clinical effect. An in-depth study, together with its pathogenesis, has been performed to clarify the underlying mechanism. Inhibitions of excessive mucus and water secretion from airway epithelium, neutrophil accumulation in the large airway, lymphocyte and macrophage accumulation around the small airway, and modulation of bacterial virulence have so far been proposed as possible mechanisms.