STUDY OBJECTIVE: Exhaled nitric oxide (NO) levels are significantly elevated in patients with inflammatory airways disorders such as asthma, and the measurement of exhaled NO has been proposed as a noninvasive marker of airways inflammation. The aim of this study was to assess the accuracy of exhaled NO levels for the diagnosis of asthma. METHODS: Two hundred forty consecutive, nonsmoking, steroid-naive patients, who were referred to our outpatient clinic with symptoms suggestive of obstructive airways disease, were investigated. Asthma was diagnosed in 160 patients on the basis of the presence of significant airways reversibility (DeltaFEV(1) > 12% predicted) and/or airways hyperresponsiveness (provocative concentration of histamine causing a 20% fall in FEV(1) < or = 8 mg/mL). Prior to lung function measurements, exhaled NO was measured during a single-breath exhalation, according to European Respiratory Society and American Thoracic Society guidelines. RESULTS: The measurement of exhaled NO in our study population showed, at a cutoff level of 16 parts per billion, a specificity for the diagnosis of asthma of 90% and a positive predictive value of > 90%. CONCLUSIONS: These findings suggest that the simple and absolutely noninvasive measurement of exhaled NO can be used as an additional diagnostic tool for the screening of patients with a suspected diagnosis of asthma.