Pulmonary endocrine cells and immunocompetent cells from the lungs of 35 subjects with bronchiectasis and in 10 normal controls were investigated by using immunohistochemical technique. The morphological and immunohistochemical characteristics of bronchus associated lymphoid tissue (BALT) of the lung were also studied. The number of calcitonin and serotonin immunoreactive pulmonary endocrine cells increased significantly in bronchiectasis as compared with those from control subjects. The number of IgA, IgG and IgM positive cells and UCHL1 positive cells were higher in the bronchial lamina propria in bronchiectasis than those in controls. These changes were most marked in hyperplastic BALT areas, which suggest that neuroendocrine and immune mechanisms may be involved in the pathogenesis of bronchiectasis.