The level of exhaled nitric oxide (eNO) is elevated in patients with asthma and eNO may be involved in airway inflammation. Exposure to allergen in sensitized individuals may contribute to airway inflammation. Our aim was to investigate the relationship between eNO, sensitization, and exposure to indoor allergen in nonsmoking adults with asthma who are not taking inhaled steroids. In subjects with a positive methacholine challenge (PD20 < 4 mg) we measured eNO (LR 2000 chemiluminesence analyzer); serum total and specific IgE; skin test to mite, cat, and dog; and allergen levels in domestic dust (Der p 1, Fel d 1, and Can f 1). Subjects were classified as exposed or not exposed to allergen according to previously proposed significant levels (> 2 micrograms/g Der p 1, > 8 micrograms/g Fel d 1, and > 10 micrograms/g Can f 1). Of the 43 subjects (> 95% atopic) complete data were available for 38, of whom 26 were both sensitized and exposed to one or more allergen and 12 were sensitized but not exposed to any allergens. eNO was significantly higher in those subjects who were both sensitized and exposed to indoor allergen than in those who were sensitized but not exposed (GM and 95% CI: 17. 69 [14.1- 22.15] versus 9.09 [6.5-12.7], p = 0.001). Levels of eNO are significantly higher in patients with asthma who are both sensitized and exposed to relevant allergen than in those who were sensitized but not exposed. eNO may be a marker of the airway inflammation induced by domestic exposure to allergen in sensitized patients with asthma.