Nitric oxide (NO) is an endogenous, soluble gas whose highly reactive molecules are involved in many biological and pathophysiological organic processes. It is generated from arginine through the action of the enzyme nitric oxide synthase (NOS). Over the recent years, accurate and non-invasive diagnostic methods have been searched for illustrating the allergic airway inflammation. Lately, great hopes have been associated with the use of measurements of exhaled nitric oxide (FENO) in the estimation of bronchial inflammation in asthmatic subjects. This method is non-invasive and can be frequently repeated. Furthermore, positive correlation between FENO and blood eosinophilia and induced sputum in atopic children and asthmatic adults has been observed. A significant increase in FENO after specific challenge test has also been shown. Therefore, determination of this parameter can be an additional source of information useful in the interpretation of occupational challenge tests in cases with boderline functional changes.