Air pollution may enhance the airway response of asthmatic subjects to allergen inhalation. To test the hypothesis that sulphur dioxide and nitrogen dioxide alone or in combination could have a contributory role, we have studied the effect of 6 h exposure to air, 200 parts per billion (ppb) sulphur dioxide, 400 ppb nitrogen dioxide, and the two gases together on the airway response to inhaled allergen in ten volunteers with mild atopic asthma. The subjects were exposed to the gases in random order at weekly visits, then challenged with pre-determined concentrations of Dermatophagoides pteronyssinus allergen 10 min after each exposure. The forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), and cumulative breath units (CBU) of D pteronyssinus allergen required to produce a 20% fall in FEV1 (PD20FEV1) were measured after each exposure. Compared with air, neither sulphur dioxide nor nitrogen dioxide nor the combination significantly altered FEV1 or FVC. Although the decreases in PD20FEV1 after exposure to each agent alone were not significant (41.2%, p = 0.125 after nitrogen dioxide; 32.2%, p = 0.506 after sulphur dioxide) the decrease after exposure to the combination was significant (60.5 [SE 8.1]%, p = 0.015). Exposure to a combination of sulphur dioxide and nitrogen dioxide in concentrations that could be encountered in heavy traffic enhances the airway response to inhaled allergen, possibly as a result of previous airway inflammation.