Recently a method to measure nitric oxide (NO) concentration in exhaled air has been developed. The method is non-invasive and easy to perform and it provides information on a fascinating molecule, with such extensive respiratory functions, ranging from bronchial and vascular dilation to ciliary motion and antibacterial defense. Nasal and sinus cavities are the site of major NO production, followed by airway and alveolar compartment. A very low nasal NO production is associated with ciliary dyskinesia, a disease characterized by severe chronic sinusitis and bronchiectasis. An increased concentration of NO in exhaled air has been reported in airway diseases, characterized by airway inflammation, such as bronchial asthma, where its concentration is related to bronchial hyperresponsiveness and sputum eosinophilia. Exhaled NO concentration in asthma is a sensitive marker of airway inflammation that reacts rapidly in response to treatment or exacerbation of disease. Clinical application of exhaled NO measurement include monitoring compliance and response to treatment, disease activity, diagnosis of asthma, and the prediction of acute exacerbations. Exhaled NO concentration may be increased also in other diseases, as COPD, bronchiectasis and some connective tissue diseases (SLE and systemic sclerosis). An increased NO production from alveolar source has been shown to be involved in oxygenation impairment of patients with liver disease, particularly in case of hepato-pulmonary syndrome.