onventional oral spirometry is a commonly used test for respiratory functions. However, the nasal passages are the primary pathway for regulating ventilation and modulating ventilated air. Here, we tested the validity of using the nasal route (nasal spirometry) for the evaluation of respiratory functions. 250 healthy young adults (150 males and 100 females; 17 to 23 years of age) were subjected to two spirometry tests: oral spirometry by using a mouth piece and nasal spirometry by using a face mask. Measurement parameters included: Vital capacity (VC), forced vital capacity (FVC), forced expiratory volume first second (FEV1), FEV1/FVC%, Forced expiratory flow (FEF25/75%). and maximum voluntary ventilation (MVV). In both males and females, only VC was significantly higher in nasal than oral spirometry, while FVC, FEV1, FEV1/FVC % FEF25/75% and MVV were significantly higher in oral than nasal spirometry. Prediction equations for different measurements of nasal spirometry were derived by multiple regression analysis using sex, height, and weight as independent variables. We conclude that nasal spirometry could be a valid procedure which may be more real in expressing normal respiratory functions.