Bosentan is an established therapy in pulmonary arterial hypertension, which may be diagnosed when the mean pulmonary arterial pressure is >25mmHg at rest or >30mmHg at exercise. Bosentan has shown efficacy, if mean pulmonary arterial pressure was elevated at rest. However, there are no data available of the effects of such a therapy, if the mean pulmonary arterial pressure is normal at rest but elevated during exercise. The purpose of this study is to provide additional data on this group of patients. Our hypothesis is that patients with exercise-induced pulmonary hypertension may show a gradual worsening of pulmonary hemodynamics and potentially a development of manifest pulmonary arterial hypertension (elevated resting pulmonary arterial pressure values) without treatment, while the introduction of Bosentan may improve hemodynamics and exercise capacity in this patient group.