Nitric oxide (NO) is a molecule with a homeostatic role in a number of physiological processes. Concerning respiratory diseases, exhaled nitric oxide (FE(NO)) is a marker of airway inflammation, and its measurement can be easily performed. To date, a large number of publications defined the performances of FE(NO). Studies on asthma attributed to FE(NO) an important role in diagnosis and especially in monitoring the effects of anti-inflammatory treatment, which label it as an "inflammometer" to be used as a guide in therapy algorithms. Less consistent results were thus far obtained in chronic obstructive lung disease, in which FE(NO) levels seem usually higher than in healthy subjects but lower than in asthma, unless an eosinophil inflammation is present, and in rhinosinusitis, where the levels of nasal NO (nNO) are low, probably because of a reduced NO flow into the nose due to mucosal swelling, while they increase after an effective treatment. Among other respiratory disorders, such as cystic fibrosis and primary ciliary dyskinesia, nNO levels are particularly low in the latter (possibly for the trapping and altered NO metabolism caused by dense secretions, and by decreased NO synthase activity) and suggest nNO as a good screening tool for such disease.