Nasal nitric oxide (nNO) is produced in high quantity in the upper airways. It is thought to be involved in host defence functions and regulation of mucociliary function, and to serve as a biochemical airborne transmitter. The measurement of nNO is easy and non-invasive. It has evolved as a screening test to exclude primary ciliary dyskinesia (PCD) in patients with suggestive symptoms, because nNO is extremely low in this condition. Nasal NO is also altered in other nasal, sinus and pulmonary pathologies, but is without diagnostic value outside of PCD.