It is now 20 years since the late Freddy Hargreave and colleagues developed a valid, safe and feasible technique to assess airway inflammation using induced sputum.1–3 At around the same time Kjell Alving and colleagues reported that the concentration of exhaled nitric oxide (FE_NO) is raised in patients with asthma.4 There has since been huge research interest in these techniques over the last 20 years and both are increasingly used in clinical practice. This has lead to a number of key and often unexpected observations which have changed the way we think about inflammatory airway disease. First, the presence of eosinophilic airway inflammation, regarded as a sine qua non of asthma, is neither closely related to the pattern nor to the severity of airway dysfunction or symptoms.5–7 Second, the presence of eosinophilic airway inflammation is more closely associated with a positive response to corticosteroids than other more traditional clinical measures, irrespective of the pattern of disease it occurs in.7–10 Thus, if the clinical question is should a patient with symptoms suggesting airway disease receive corticosteroid treatment (and it often is), then the identification of eosinophilic airway inflammation would be a better basis for making this decision than categorising the patient or applying other tests. Third, a raised FE_NO level appears to be just as good an indicator of a positive response to corticosteroids in a heterogeneous …