Exhaled nitric oxide fraction (F_ENO) values can be easily measured using portable analysers and are a surrogate marker of airway eosinophilia.F_ENOmay be useful in diagnosing and monitoring conditions characterised by airway eosinophilia,i.e.asthma and possibly COPD. Many factors other than asthma and COPD affectF_ENO, especially atopy, which is associated with elevatedF_ENO. One guideline recommends thatF_ENOshould be used as part of the diagnostic pathway for asthma diagnosis in adults and children aged >5 years. The role ofF_ENOin monitoring asthma is even less clear, and most guidelines do not recommend its use outside of specialist asthma clinics. Currently,F_ENOis not recommended for diagnosis or monitoring of COPD. AlthoughF_ENOis starting to find a place in the management of asthma in children and adults, considerably more research is required before the potential ofF_ENOas an objective measurement in asthma and COPD can be realised.Key pointsFor individuals aged ≥12 years,F_ENOis not recommended by all guidelines as a test to diagnose asthma (recommended only by the UK National Institute for Health and Care Excellence guideline for asthma symptoms, which are likely to respond to corticosteroid treatment).F_ENOmay be used in conjunction with other investigations to diagnose asthma in 5–16-year-olds where there is diagnostic uncertainty, but further evidence is required.F_ENOis not recommended as a routine test to monitor all patients with asthma or to titrate asthma treatment.F_ENOis not recommended for routine clinical testing in adults with COPD.F_ENOmay be useful to identify patients with COPD who could benefit from the use of inhaled corticosteroids (asthma–COPD overlap).Educational aimsTo understand what factors other than asthma and COPD affectF_ENOTo understand the current controversies in the application ofF_ENOto diagnosis and management of asthma in childrenTo understand the current controversies in the application ofF_ENOto diagnosis and management of asthma and COPD in adults