Cytological studies were performed on bronchoalveolar lavage(BAL) fluid in 7 cases with hypersensitivity pneumonitis, 6 with sarcoidosis, 6 with chronic bronchitis and 5 with bronchiolitis. In the cases with hypersensitivity pneumonitis and bronchiolitis, cytological studies of BAL fluid were compared with the histological findings of lungs. Furthermore, animal models of hypersensitivity pneumonitis were made and examined likewise. The results were as follows. 1) In the cases with hypersensitivity pneumonitis, a marked increase in the lymphocyte count in BAL fluid was characteristic. These cases showed an increase in the percentage of T lymphocytes in BAL fluid as compared with healthy subjects. In the cases with sarcoidosis, an increase in the number of lymphocytes was also found and especially the ratio of the percentage of T lymphocytes to that of B lymphocytes was much higher than those of the other diseases and healthy subjects. 2) An increased number of neutrophils in BAL fluid was observed in the cases with chronic bronchitis. In the cases with bronchiolitis, a marked increase in the neutrophil count was characteristic. 3) A common histological finding in hypersensitivity pneumonitis was mononuclear cell infiltration into alveolar septa and most of the cells in BAL fluid were lymphocytes. In bronchiolitis, mononuclear cell infiltration into and around the bronchiolar walls was found commonly and neutrophils were found increased in BAL fluid. 4) In experimental hypersensitivity pneumonitis, mononuclear cell infiltration into septa closely paralleled the lymphocyte appearance in BAL fluid.