Asthma is a pathologically heterogeneous disease, and the phenotype is characterized by different types of airway inflammation. Exhaled nitric oxide (F(E)NO) measurements are a surrogate marker specific for eosinophilic airway inflammation. The latter is usually associated with steroid responsiveness, and hence, F(E)NO may be used to guide steroid requirements in certain clinical situations. High F(E)NO levels may be used to predict likely benefits with inhaled corticosteroid (ICS) therapy. Both high and low F(E)NO levels are prognostically significant when withdrawal of ICS treatment is being considered. Studies have shown that, just as for induced sputum, repeated F(E)NO measurements improve the cost-effectiveness of ICS therapy when used to guide dose requirements. In practice, F(E)NO measurements are useful in the management of severe or difficult asthma. High and low F(E)NO levels in symptomatic patients provide the clinician with information that enables active eosinophilic airway inflammation to be included or excluded. Either outcome is helpful in decision making. F(E)NO measurements complement the use of other tests in asthma, but more work is required to determine reference values and cut-points for appropriate interpretation.