Eleven mild atopic asthmatic patients were exposed for 6 h, in randomized order, to air, 100 ppb O3, 200 ppb NO2, and 100 ppb O3 + 200 ppb NO2, followed immediately by bronchial allergen challenge. Subsequently 10 of these patients were exposed for 3 h to air, 200 ppb O3, 400 ppb NO2, and 200 ppb O3 + 400 ppb NO2, followed immediately by bronchial allergen challenge. All exposures were carried out in an environmental chamber, with intermittent moderate exercise, and a minimal interval of 2 wk. Exposure for 6 h to 100 ppb O3, 200 ppb NO2, and 100 ppb O3 + 200 ppb NO2 did not lead to any significant increase in the airway response of these individuals to inhaled allergen, when compared with exposure for 6 h to air. In contrast, exposure for 3 h to 200 ppb O3, 400 ppb NO2, and 200 ppb O3 + 400 ppb NO2 significantly decreased the dose of allergen (in log cumulative breath units [CBU]) required to decrease FEV1 by 20% (allergen PD20FEV1), compared with exposure to air (geometric mean CBU: 3.0 for air versus 2.66 for O3 [p = 0.002]; 2.78 for NO2 [p = 0. 018]; 2.65 for O3 + NO2 [p = 0.002]). These results suggest that the pollutant-induced changes in airway response of mild atopic asthmatics to allergen may be dependent on a threshold concentration rather than the total amount of pollutant inhaled over a period of time.