A comprehensive investigation of the morphology of human airway epithelial tight junctions was carried out by freeze-fracture electron microscopy using quantitative methods designed to analyze a range of junctional characteristics. Extrapulmonary bronchi that appeared grossly normal were taken at sites distant from tumor in lungs resected for pulmonary carcinoma. The absence of cellular atypia in the samples was confirmed by histology. Airway levels I (main bronchus; n = 7 subjects) and II (lobar bronchus; n = 5 subjects) were compared with respect to junctional depth, strand number, and junctional complexity. Junctional complexity was assessed by frequency of strand interconnection and numbers of strands per interconnection. Comparisons between airway levels I and II for these parameters showed that there were no significant differences in strand number or junctional complexity between the two airway levels. However, junctional depth was slightly but significantly reduced at level II compared with level I (P less than 0.01). The arrangement of strands varied considerably from one junction to the next, irrespective of the cell types involved. "Parallel" and "network" patterns of junctions were observed; the existence of gradations between these two patterns indicated that they represent opposite extremes of a single junctional form rather than distinct categories of junction. These results have allowed us to establish a data pool for normal human bronchi from which the structure of epithelial cell junctions in bronchial diseases can be compared.