Cytological studies were performed on bronchoalvelar lavage(BAL) fluid in 40 cases with bronchial asthma. The results were as follows. 1) In 13 healthy subjects the cells in BAL fluid comprised 89.6% macrophages, 9.7% lymphocytes, 0.7% neutrophils, 0.4% eosinophils and 0% basophils. 2) In asthmatic cases the cells in BAL fluid comprised 54.4% macrophages, 12.6% lymphocytes, 11.8% neutrophils, 21.2% eosinophils and 0.1% basophils. These cases showed a marked increase in the number of eosinophils and a moderate increase in the neutrophil count as compared with healthy subjects. The increased number of eosinophils in lavage fluid was relatively closely related to positive immediate skin tests, high serum IgE levels and peripheral eosinophilia. As for asthmatic types, this was marked in the atopic type. By contrast, an increased number of neutrophils in the lavage fluid was frequently found in non-atopic or intractable asthmatics showing the frequent complication of peripheral airway disorder in pulmonary function tests. Therefore, it seems that the present study gave much information on the characteristics and the etiology of the intractable asthmatics. 3) Eosinophils in BAL fluid reflected the appearance of eosinophils in the alveolar spaces and eosinophil infiltration into the septa in the histological findings. On the other hand, neutrophils could be observed clearly by the BAL method even when they could not be observed clearly by histological studies. In conclusion, the cytological studies performed on BAL fluid are considered useful in the diagnosis of bronchial asthma, especially for the classification into subtypes of asthma.