Inhaled corticosteroids are known to reduce components of the airway inflammation characteristic of asthma and improve airway hyperresponsiveness. However, the effect of inhaled corticosteroids on ozone-induced airway responses is unknown. Eight dogs inhaled budesonide [2.74 +/- 0.25 (SE) mg/day] or lactose powder twice daily for 7 days before inhaling ozone (3 ppm for 30 min) or dry air. Acetylcholine airway responsiveness was measured before and 1 h after ozone, followed by a bronchoalveolar lavage (BAL). The response to acetylcholine was expressed as the concentration causing an increase in lung resistance of 5 cmH2O.l–1.s above baseline (acetylcholine provocation concentration). Budesonide pretreatment significantly attenuated the ozone-induced increase in pulmonary resistance (P = 0.003) and neutrophil influx into BAL (P = 0.001) and significantly reduced BAL eosinophils (P = 0.026). However, budesonide pretreatment had no significant effect on ozone-induced airway hyperresponsiveness. After budesonide, the acetylcholine provocative concentration fell from 5.96 mg/ml (%SE 1.46) before to 1.11 mg/ml (%SE 1.63) after ozone (P = 0.006). After lactose, the acetylcholine provocative concentration fell from 5.34 mg/ml (%SE 1.40) before to 0.50 mg/ml (%SE 1.85) after ozone (P = 0.001). Dry air inhalation did not cause airway hyperresponsiveness (P = 0.68). These results suggest that ozone-induced airway hyperresponsiveness is steroid resistant and that airway neutrophils or eosinophils are not important in its pathogenesis.