Histological investigations of lung specimen and bronchial membrane from patients with bronchial asthma and hypersensitivity pneumonitis were performed by the technique of transbronchoscopic biopsy. The results are summarized as follows. 1) Histological changes such as interstitial and peribronchiolar mononuclear cell infiltration, and septal thickening were observed in lung specimens from patients with bronchial asthma. Histological changes of bronchial membrane were also observed in these cases. 2) In patients with bronchial asthma, histological changes in bronchiolar-alveolar regions were closely related to age, positive immediate skin reaction and precipitin to Candida, serum IgE level and micronodular shadow on chest X-ray film. The degree of histological changes in segmental bronchus were related to age at onset and positive immediate, late, and delayed skin reaction to Candida. Asthmatic patients with marked histological changes in segmental bronchus showed more decreasing ratio of FVC, FEV 1.0%, MMF, V50 and V25 than the other cases. 3) Granulomatous change in one case and Masson body in another were observed, alveolitis in three and peribronchiolar cell infiltration in four of five patients with hypersensitivity pneumonitis.