Background: Increased ventilatory response has been shown to have a high prognostic value in patients with chronic heart failure. Our aim was therefore to determine the ventilatory efficiency in pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension by cardiopulmonary exercise testing (CPET) identifying PH-patients with increased risk for death within 24 months after evaluation. Methods: 116 patients (age: 64 +/- 1 years) with a mean pulmonary arterial pressure of 35 +/- 1 mmHg underwent CPET and right heart catheterization. During a follow-up of 24 months, we compared the initial characteristics of survivors (n = 87) with nonsurvivors (n = 29). Results: Significant differences (p <= 0.005) between survivors and nonsurvivors existed in ventilatory equivalents for oxygen (42.1 +/- 2.1 versus 56.9 +/- 2.6) and for carbon dioxide (Ve/VCO2) (47.5 +/- 2.2 versus 64.4 +/- 2.3). Patients with peak oxygen uptake <= 10.4 ml/min/kg had a 1.5-fold, Ve/VCO2 >= 55 a 7.8-fold, alveolar-arterial oxygen difference >= 55 mmHg a 2.9-fold, and with Ve/VCO2 slope >= 60 a 5.8-fold increased risk of mortality in the next 24 months. Conclusions: Our results demonstrate that abnormalities in exercise ventilation powerfully predict outcomes in PH. Consideration should be given to add clinical guidelines to reflect the prognostic importance of ventilatory efficiency parameters in addition to peak VO2.