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Pulmonary hypertension is a clinical syndrome characterized by a progressive increase in pulmonary vascular resistance leading to right ventricular failure and death. Pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) are key subgroups of this disorder with comparable clinical and pathological findings. Resting pulmonary haemodynamics correlate only moderately with functional parameters and do not predict prognosis in these patients sufficiently accurately. We therefore correlated exercise haemodynamics with peak oxygen uptake (peakVO2) and determined their prognostic significance.Thirty-six consecutive patients (21 female, 54 ± 15 years) with PAH (n = 21) or inoperable CTEPH were studied. The mean follow-up period was 1709 ± 837 days. All patients underwent right heart catheterization at rest and during exercise, and cardiopulmonary exercise testing. Patients had severe pulmonary hypertension at rest (mean pulmonary artery pressure 46 + 11 mmHg, cardiac index 2.2 ± 0.6 L/min/m2, pulmonary vascular resistance 861 ± 330 dynes/s/cm5). Exercise cardiac index correlated with peakVO2 (r = 0.59, P < 0.001) and was the only independent predictor of peakVO2 on multivariate stepwise linear regression analyses (P < 0.001). PeakVO2 was the strongest predictor of survival (χ2 = 14.5, P = 0.003). Among haemodynamic variables, only exercise cardiac index (χ2 = 5.6, P = 0.018) and the slope of the pressure/flow relationship (χ2 = 4.1, P = 0.04) were significant prognostic indicators.The ability of the right ventricle to increase the cardiac index during exercise is an important determinant of exercise capacity in patients with pulmonary hypertension. It also predicts prognosis and might therefore be useful in the clinical assessment of these patients.