The differences in two groups of patients (24 with chronic bronchitis and 24 with bronchial asthma) were compared by bronchoscopy and bronchial biopsies. The same investigations were carried out on 24 healthy control subjects. The only bronchoscopic difference between the two groups was hypertonicity of the bronchial wall, which was frequent in the asthmatics and absent in the chronic bronchitics. Mucus was scanty and viscous in the asthmatics and more abundant and watery in the chronic bronchitics. Hypersecretion of mucus occurred in both groups. In asthma the mucous hyperplasia affected chiefly the goblet cells. In chronic bronchitis the deeper bronchial glands were affected as well as the goblet cells. In asthma the mucus stained predominantly with the periodic-acid Schiff technique, and was thus presumed to contain much neutral mucopolysaccharide. In chronic bronchitis the mucus was strongly alcian blue positive and was thought to consist largely of acid mucopolysaccharide. Thickening of the basement membrane was almost constant in bronchial asthma and rare in chronic bronchitis. Tissue eosinophilia was very frequent and intense in asthma but infrequent and sparse in chronic bronchitis. Mast cells were degranulated and decreased in number in asthma but increased in number and packed with granules in chronic bronchitis. It is presumed that there is a relation between tissue eosinophilia and degranulation of mast cells. The study confirms that bronchial asthma and chronic bronchitis are two distinct diseases, both with a separate pathology.