Upper airway cough syndrome (UACS), the most common cause of prolonged cough, is diagnosed based on clinical findings without specific diagnostic test. The concentration of nitric oxide in nasal cavity air (nNO) is influenced by allergic rhinitis and/or sinusitis, both of which are common causes of UACS. We measured nNO levels in patients with UACS and those with other causes. We also examined the usefulness of measuring nNO for differentiating patients with sinusitis from those without sinusitis. The study included 93 adult patients with prolonged cough lasting more than threeweeks. Etiologies of cough were identified and nNO was measured at the initial investigation. UACS was diagnosed in 58 patients (62.4%), and sinusitis was identified in 11 (19.0%) of the 58 patients with UACS. Levels of nNO in UACS did not differ from non-UACS etiologies (316.2±129.2 vs. 334.9±88.2 ppb; p=0.452), suggesting that the measurement of nNO could not discriminate UACS from other etiologies of prolonged cough. However, patients with sinusitis showed significantly decreased nNO levels (190.1±114.8ppb) compared with patients with UACS without sinusitis (345.7±114.6ppb; p<0.001) and non-UACS patients (334.9±88.2 ppb; p<0.001). In a receiver operating characteristic curve analysis for the diagnosis of sinusitis in prolonged cough, the best sensitivity (73.2%) and specificity (81.8%) were obtained with a nNO cutoff value of 279.0 ppb. These findings imply that the measurement of nNO could be useful for diagnosis of prolonged cough associated with sinusitis.