Background: Exhaled nitric oxide (FeNO) is a well described marker of airway inflammation in asthma and is also known to increase after chronic exposure to inhaled allergens. It is not known whether monitoring FeNO could be useful during food challenges to detect early or subclinical reactions. Methods: Forty children aged 3 to 16 years undergoing an allergen-food challenge at two centres were prospectively recruited for this study. FeNO was assessed before and repeatedly after the food-challenge. Results: Data were obtained from a total of 53 challenges (16 positive, 37 negative) and were compared between the two groups. Half of the patients with a positive food challenge exhibited clinical upper respiratory symptoms. The FeNO significantly decreased in 7 of 16 patients with a positive challenge test within 60 to 90 minutes after the first symptoms of an allergic reaction. Conclusion: Our results show a significant decrease in FeNO after a positive food challenge suggesting involvement of the lower airways despite absence of clinical and functional changes of lower airways. Prospective blinded studies are needed to confirm these results.