Mild excerice in 7 patients with upper airway obstruction but without diffuse lung disease caused a mean decrease in arterial oxygen tension of 11 mm Hg. Exercise hypoxemia disappeared after surgical removal of obstruction in 3 patients tested. Exercise hypoxemia due to relative alveolar hypoventilation was observed in 4 normal subjects with external combined inspiratory and expiratory resistance. Analysis of mechanics of air flow through an orifice suggests that exertional dyspnea is caused by manifold increase of airway resistance during exercise; acute respiratory failure might be precipitated by further minimal reduction in airway lumen once it has reached a diameter of 8 mm. Clinicians should be alert to the possibility of upper airway obstruction in any symptomatic patient who has had tracheal intubation or in patients with obscure wheezing, especially on exercise.