Diffuse panbronchiolitis (DPB) is a disease with chronic inflammation exclusively located in the region of respiratory bronchioles. The pathologic features of the disease are characterized by thickening of the wall of the respiratory bronchiole with infiltration of lymphocytes, plasma cells and histiocytes, and extension of the inflammatory changes toward peribronchiolar tissues. In the advanced stage, secondary ectasia of proximal bronchioli may occur. These changes appear as diffusely disseminated small nodular shadows throughout both lungs on the chest roentgenogram. Obstructive respiratory functional impairment, occasional symptoms of wheezing, and also cough and sputum resemble the feature of emphysema, bronchial asthma, or chronic bronchitis, respectively. In the advanced stage, large amounts of purulent sputum and dilatation of proximal terminal conducting bronchioli resemble bronchiectasis. However, diffuse panbronchiolitis belongs to a distinctly different category from these diseases, and should be distinguished from them, because it may often show rapid progression with fatal outcome. The disease is dominant in males and the onset is unrelated to age. More than 1,000 cases of probable diffuse panbronchiolitis and 82 histologically-confirmed cases have been collected in Japan.