Air pollution constitutes an important factor for asthma aggravation, and there is increased concern about respiratory health effects of common air pollutants. The purpose of this study was to examine how exposure to a high ambient concentration nitrogen dioxide (NO2) prior to a bronchial allergen challenge modulated the inflammatory response in the bronchi. Thirteen subjects with mild asthma and allergy were exposed at rest to either purified air or 500 microg x m 3 NO2 for 30 min, followed 4 h later by an allergen inhalation challenge. The exposures (NO2 or air) were performed in random order and at least 4 weeks apart. Lung function during NO2/air exposure and allergen challenge was measured by plethysmography, and then hourly by portable spirometry after exposures. Subjective symptoms were recorded during and after exposure. Bronchoscopy with bronchial wash (BW) and bronchoalveolar lavage (BAL) was performed 19 h after allergen challenge. NO2+allergen enhanced the percentage of neutrophils in both BW and BAL compared to air+allergen (BW 19 vs. 11, P=0.05; BAL 3 vs. 1, P=0.02 median values). The levels of eosinophil cationic protein (ECP) in BW was higher after NO2+allergen compared to air+allergen (90 vs. 3.6 microg/l; P=0.02, median values). There was no NO2-associated effect on symptoms or pulmonary function. These data suggest that ambient NO2 can enhance allergic inflammatory reaction in the airways without causing symptoms or pulmonary dysfunction.