Diffuse panbronchiolitis (DPB) is known as a chronic progressive inflammation of the peripheral airway followed by lethal respiratory failure. Bronchoalveolar lavage (BAL) was performed to clarify the pathogenesis of DPB compared to chronic bronchitis. The recovery rate of BAL fluid in DPB decreased, but total cell count increased enormously. Furthermore, increased proportions of neutrophils in BAL fluid from DPB patients was characteristic and included a relative decrease in alveolar macrophages. Patients with DPB were classified based on the presence or absence chronic airway infection with Pseudomonas aeruginosa. There was no significant difference in cellular components of BAL fluid among the patients with or without Pseudomonas infection. Patients with DPB showed both obstructive and restrictive ventilatory disturbances. There was no correlation between parameters of these respiratory functions and total cell counts or various cellular proportions in BAL fluid. Infiltrating cell density around bronchioles of patients with DBP was measured. Significantly higher cell density was shown in patinets with DPB compared to other disease controls. There were also significant negative correlations between cell density and respiratory parameters such as % vital capacity, % V50 and % V25. These results indicate that remarkable increase in neutrophils in BAL fluid and the marked cell infiltration around bronchioles in DPB patients could play a crucial role in the pathogenesis of DPB.