Higer than normal nitrite oxide level is released in several inflammatory processes including asthma. It is seen in asthmatic conditions that have eosinophilic proliferation. Thus, it is used as a surogate marker for active eosinophilic inflammation in the airway. Those conditions would normally reponse to inhaled steroid and other biologic agents. FeNO measurement is an important tool added-on to peak flow and lung function test to mornitor the progess of asthma, its responsiveness to medications and also in the diagnosis of asthma itself.