The maximal oxygen uptake and anaerobic threshold can be determined in patients with pulmonary vascular disease from noninvasive gas exchange measurements during progressive exercise. Maximal O2 utilization correlates directly with the maximal level of cardiac output and is inversely proportional to the pulmonary vascular resistance and the level of pulmonary arterial pressure. The accuracy of predicting the hemodynamic profile from noninvasive gas exchange measurements will obviously improve as more such patients are evaluated. Therefore, the determination of VO2 max has the potential of becoming a practical screening technique for detecting and predicting the severity of pulmonary vascular disease.