To examine the mechanism of tissue damage which causes bronchiolectasis in diffuse pan bronchiolitis (DPB), the cellular components, elastase and its main inhibitor, alpha 1-protease inhibitor (al-PI) were measured in bronchoalveolar and bronchial lavage fluid (BALF and BLF) from 14 DPB patients. A predominant increase in the neutrophil count was observed in DPB. Elastase activity in BALF and BLF was about 1,000-fold higher in the DPB group than in the control group. An inhibitor study and a positive correlation between elastase activity and the neutrophil count in both lavage fluids from the DPB group indicated that the activity was mainly that of neutrophil elastase. Western blot analysis of al-PI showed that most of the al-PI in the lavage fluids from DPB group was degraded. These results indicated that neutrophil infiltration increases the level of elastase in the DPB lesions; this increase seems to be closely related to tissue damage. (Internal Medicine 31: 599-605, 1992)