Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine pulmonary tissue from 9 patients with cystic fibrosis (CF), 12 patients with diseases other than CF, and from two surgically resected specimens with no known airways disease. A region of the human airways, the transition zone, was observed between the end of the terminal bronchiole and the type II alveolar cell lining of the respiratory bronchioles. This region was lined predominantly by nonciliated bronchiolar (NCB) cells. Patients with CF exhibited indistinct transition zones, epidermoid metaplasia, large dilated bronchial glands, copious surface mucus, alveolar destruction, and unusual microvilli; no single lesion specific for cystic fibrosis was identified.