In addition to being an air pollutant, nitrogen dioxide (NO₂) is a potent inflammatory oxidant generated endogenously by myeloperoxidase and eosinophil peroxidase. In these studies, we sought to determine the effects of NO₂ exposure on mice with ongoing allergic airway disease pathology. Mice were sensitized and challenged with the antigen, ovalbumin (OVA) to generate airway inflammation and subsequently exposed to 5 or 25 parts per million (ppm) NO₂ for 3 days, or 5 days followed by a 20 day recovery period. While 5 ppm NO₂ elicited no pathologic changes, inhalation of 25 ppm NO₂ alone induced acute lung injury, which peaked after 3 days and was characterized by increases in protein, lactate dehydrogenase (LDH), and neutrophils recovered by bronchoalveolar lavage (BAL), as well as lesions within the terminal bronchioles. Importantly, 25 ppm NO₂ was also sufficient to cause airways hyperresponsiveness (AHR) in mice, a cardinal feature of asthma. The inflammatory changes were ameliorated after 5 days of inhalation and completely resolved after 20 days of recovery following the 5 day inhalation. In contrast, in mice immunized and challenged with OVA, inhalation of 25 ppm NO₂ caused a marked augmentation of eosinophilic inflammation and terminal bronchiolar lesions, which extended significantly into the alveoli. Moreover, 20 days post cessation of the 5 day 25 ppm NO₂ inhalation regimen, eosinophilic and neutrophilic inflammation, pulmonary lesions, and AHR were still present in mice immunized and challenged with OVA. Collectively, these observations suggest an important role for NO₂ in the airway pathologies associated with asthma, both in the modulation of the degree and the duration of the inflammatory response, as well as in the induction of AHR.