BACKGROUND The association of asthma with sensitization and allergen exposure is known to be complex. There have been few studies of bronchial responsiveness in relation to both risk factors in adults. OBJECTIVE To determine the relation of bronchial responsiveness to allergen exposure and IgE sensitization in a community study taking into account the major determinants of bronchial responsiveness in adulthood. METHODS Cross-sectional data were drawn from 1,884 participants in 20 centers in the European Community Respiratory Health Survey follow-up, which included measurement of house dust mite and cat allergen in mattress dust samples, and IgE sensitization to four allergens. Bronchial responsiveness to methacholine was expressed as a continuous variable, and analyzed by multiple regression. RESULTS The trend toward greater bronchial responsiveness with increasing exposure to cat allergen was greater in those sensitized to any of the four allergens than those not sensitized (p = 0.001); there was no significant interaction between cat sensitization and Fel d 1 exposure. No trend was found with house dust mite allergen exposure. The difference in bronchial responsiveness between those exposed to the highest levels compared with the lowest was approximately -2.02 doubling doses of PD20 (95% confidence interval, -3.06 to -0.97), and nearly as great in those exposed to more moderate levels. CONCLUSIONS Cat allergen exposure at moderate levels may be harmful to all atopic adults. The clinical implication is that it is insufficient to test patients with asthma for cat sensitization; all atopic individuals may benefit from reduced cat exposure.