The aim of this study was to examine the process underlying the increase in responsiveness of human bronchus related to sensitization status by examining inflammatory cells in sensitized and nonsensitized tissue. Sensitization status was determined in tissue from 26 subjects by detecting the presence or absence of contraction of bronchial rings to common Australian allergens. Serial sections (6-micron) of bronchial tissue were stained for mast cells, T lymphocytes, and eosinophils. There was no significant difference between the total number of inflammatory cells present in all locations (to a depth of 0.5 mm) of nonsensitized (n = 14) and sensitized (n = 12) bronchi. Similar numbers of cells were present in the two patient groups in epithelium, lamina propria, and adventitia. In contrast, there were significantly (p = 0.03) more inflammatory cells present within the smooth muscle of sensitized bronchi. A significantly (p = 0.04) greater number of mast cells in sensitized (9 [2-14]) versus nonsensitized (2[0-8]) tissue was found in this location (data are median with first and third quartile range). In addition there was a trend toward a greater area of smooth muscle in sensitized tissue. There results suggest that an interaction between smooth muscle and inflammatory cells, especially mast cells, may play a role in bronchial hyperresponsiveness in vitro.