Although asthma is a common respiratory condition, there is no standardised definition of the type, severity or frequency of symptoms, nor of the findings on investigation. Therefore, it is not possible to make evidence-based recommendations on how to diagnose patients with asthma. This article describes the use of the fraction of exhaled nitric oxide (FeNO) as a biomarker of airway inflammation, thereby assisting diagnosis of asthma in clinical practice. FeNO can also be used to monitor adherence to inhaled corticosteroid therapy.