Assessment of exhaled nitric oxide (eNO) creates new diagnostic possibilities for evaluation of the inflammatory process especially in children, in whom not all of the contemporary methods of diagnosing respiratory tract diseases are applicable. eNO concentration can be measured in children of all age, including neonates. Direct assessment of eNO is possible with an on-line method, in which NO is directly exhaled into a chemiluminescence analyser. An off-line method is performed by exhalation into a collapsible bag of nonreacting material and analysing the collected air with appropriate analysers. eNO measurement is a non-invasive, repeatable, rapid and sensitive method, producing results even during measurement. Assessment of eNO in children may be used to quantify inflammation in asthma and other respiratory tract diseases specific for this age group. Monitoring of the therapy and response to the anti-inflammatory agents i.e. glucocorticosteroids and anti-leukotriene drugs can also be based on eNO measurement.