Nitric oxide (NO) has witnessed an explosion of interest of scientists all over the world during the last decade. This small gaseous molecule is produced in many systems such as the nervous system, cardiovascular system, the upper and lower airways. In all of these it contributes to a number of (patho)physiological processes. Concerning the airways, NO concentrations in the upper respiratory tract are much higher (i.e. ranging from 200 to 2000 parts per billion (ppb)) than NO levels in the lower respiratory tract (i.e. ranging from 4 to 160 ppb). NO is most frequently measured using a chemiluminescence method, based on a reaction of NO with O3 resulting in the emission of light. In the airways NO exerts many functions in host defense, ciliary activity, inflammation and it is also an aerocrine messenger between the upper and lower airways. Nasal NO concentrations are influenced by age, physical exercise, smoking and certain drugs. Nasal NO is conveniently measured in all ages and can be used for screening of disease or monitoring the effects of treatment. Pathological conditions, as in allergic rhinitis, sinusitis, nasal polyps, cystic fibrosis and primary ciliary dyskinesia, result in altered nasal NO concentrations. The clinical relevance for measurement of nasal NO in different conditions, however, remains to be established.