The ultrastructure of the bronchial epithelium of three asthmatic girls was studied from material obtained during bronchoscopy. In all patients the large bronchi were lined with an altered pseudostratified ciliated epithelium having a severely damaged ciliary border. Injury to the ciliated cells was manifested by apical bleb formation, destruction of free kinocilia, pathological ciliogenesis, dilation of the spaces of the endoplasmic reticulum and Golgi complex, an increase in the number of vacuoles and lysosomes, and the appearance of altered mitochondria. The most outstanding feature of the epithelium was the marked hyperplasia of the goblet cells with the development of intraepithelial mucous glands. The mucous elements of the epithelium had discharged their mucus. Further, there were signs of mucus cell injury and degeneration. Squamous metaplasia of the pseudostratified epithelium was not observed.