The aim of this study was to determine whether ozone enhances bronchial responsiveness to allergens in subjects with allergic asthma, or facilitates a bronchial response in subjects with allergic rhinitis. Twenty-four subjects with mild stable allergic asthma, 12 subjects with allergic rhinitis without asthma, and 10 healthy subjects participated in the study. Subjects breathed 250 ppb ozone or filtered air (FA) for 3 h of intermittent exercise. Airway responsiveness to methacholine was determined 1 h before and after exposures, and allergen responsiveness 3 h after exposures. We determined the concentration of methacholine (PC20FEV1) and the dose of allergen (PD20FEV1) producing a 20% fall in FEV1. In the subjects with asthma, FEV1 decreased by 12.5 +/- 2.2% (mean +/- SEM; p = 0.0001), PC20FEV1 of methacholine by 0.91 +/- 0.19 doubling concentrations (p = 0.0001) and PD20FEV1 of allergen by 1.74 +/- 0.25 doubling doses (p < 0.0001) after ozone compared with FA. The changes in lung function, methacholine, and allergen responsiveness did not correlate with each other. In the subjects with rhinitis, mean FEV1 decreased by 7.8% and 1.3% when ozone or FA, respectively, were followed by allergen inhalation (p = 0.035). Therefore, our data suggest that short-term exposure to ozone can increase bronchial allergen responsiveness in subjects with mild allergic asthma or rhinitis.