[en] There has been a growing interest for exhaled biomarkers. We review studies examining NO as a potential marker of airway inflammation, enabling noninvasive repeated monitoring of airway inflammation. The measurement technique has been standardized. We have determined the local normal levels for the Liege region. The exhaled NO level is elevated in asthma, and can predict asthma exacerbation. Exhaled NO has a value for the diagnosis of cystic fibrosis and primary ciliary dyskinesia.