Morphological investigations (light microscopy, enzyme histochemistry, immunopathology) were carried out in cases with asthma bronchiale and chronic bronchitis. According to the histological picture no additional parameters were found with enzyme histochemical and immunopathological methods to specify the diagnosis asthma bronchiale and chronic bronchitis. The most important diagnostic factors in chronic bronchitis are hyperplasy of mucous bronchial glands and atrophy of serous bronchial glands with a loss of activity of acid phosphatase. Diffuse thickenings of basal membrane were more often in asthma bronchiale, localized thickenings more often in chronic bronchitis. Hypertrophy of goblet cells, infiltrations with round cells and metaplasia are found both in asthma bronchiale and chronic bronchitis in different degree. Correlations exist between the activity of leucine aminopeptidase in the stroma of the bronchus and degree of cellular infiltration with lymphocytes and plasma cells.