OBJECTIVES: The fraction of exhaled nitric oxide (FeNO), a marker of eosinophilic inflammation, may be a useful diagnostic test in asthma. This systematic review aimed to identify and synthesise evidence relating to the diagnostic accuracy of FeNO for asthma. METHODS: Systematic searches of nine key biomedical databases and trial registers (including MEDLINE, EMBASE , the Cochrane library and clinicaltrials.gov) were carried out to November 2014. Records were considered by one reviewer and included if they: recruited patients presenting with the symptoms of asthma; used a single set of criteria (i.e. not case-control); measured FeNO in accordance with American Thoracic Society guidelines, 2005 (off-line measurements excluded); reported/allowed calculation of true positive, true negative, false positive and false negative patients as classified against any reference standard. Study quality was assessed using QUADAS II. Data was extracted by one reviewer using a standardised form and checked by a second. Meta-analysis was planned where clinical study heterogeneity allowed. Rule-in and Rule-out uses of FeNO were considered. RESULTS: CONCLUSIONS: FeNO has variable diagnostic accuracy even within subgroups of studies with similar characteristics. However, FeNO could be informative within a diagnostic pathway involving other tests. Cut-off values should probably be lower in children.