Current guideline-defined asthma control cannot be applied to the level of airway inflammation because neither symptoms nor the results of basic pulmonary function tests can reflect ongoing airway inflammation.Consequently, asymptomatic or minimally symptomatic patients are usually considered to have controlled asthma even if they have subclinical airway inflammation. Measures of airway inflammation are thus required to identify patients with silent airway inflammation which does not manifest itself as symptoms or impaired lung function. In addition, measurement should be easy to perform, reproducible, and associated with a high degree of acceptance by patients. In this regard, the advent of FeNO measurements represents a significant advance in monitoring airway inflammation of asthmatic patients. High FeNO values above certain cut-point may indicate active eosinophilic airway inflammation and the likelihood of deterioration in asthma control. However, it is still unclear that high FeNO in asymptomatic patients implies the need for the anti-inflammatory treatment.