Pulmonary hypertension (PH) of various etiologies causes dyspnea, impairs exercise performance and is associated with reduced quality of life (QoL) and survival. Treatment options include therapy for any underlying causes, pulmonary vasodilator drugs, oxygen and, in selected cases, pulmonary endarterectomy or lung transplantation. Unfortunately, PH specific drugs are expensive, associated with side effects and even combined pharmacological treatment is often not sufficient to achieve clinical benefits. Therefore, novel therapeutic drugs are needed. We have recently demonstrated that sleep related breathing disorders, which are common in PH patients, can be improved by both nocturnal oxygen therapy and acetazolamide (AZA). AZA is a carbonic anhydrase (CA) inhibitor that acts as a respiratory stimulant thereby improving oxygenation and possibly PH. There are even data suggesting that CA-inhibitors have a direct pulmonary vasodilator effect. However, the potential role of AZA in the treatment of PH has not been conclusively studied. Therefore, the purpose of the current project is to investigate, the chronic clinical effects of AZA in PH patients.