Pulmonary arteriovenous malformations can result in severe hypoxemia and dyspnea. We measured pulmonary function, arterial blood gases, and hemodynamics in 10 patients with such malformations. Pulmonary-function tests were normal, but hypoxemia was associated with chronic hyperventilation at rest (mean, 12 liters per minute; mean carbon dioxide tension, 28 mm Hg). With exercise, ventilation increased more than expected for the level of carbon dioxide production. Balloon embolization of 58 of the 71 visible vascular malformations in the 10 patients resulted in an increase in arterial oxygen tension (43 vs. 64 mm Hg; P less than 0.001) and hemoglobin saturation (79 vs. 92 per cent; P less than 0.001). Nine patients had improved exercise tolerance. Forty-eight to 72 hours after correction of the hypoxemia, resting ventilation had decreased but was still above normal (mean, 9.3 liters per minutes; mean carbon dioxide tension, 29 mm Hg). We conclude that ventilatory responses in these patients are similar to those of people from sea-level areas who are acclimated to high altitudes and that dyspnea is due to inappropriately high levels of ventilation for a given workload under hypoxic conditions.