Patients with pulmonary arterial hypertension (PAH) will be treated with carvedilol for 24 weeks. During this time, carvedilol will be titrated from an initial dose of 3.125mg BID to a maximal dose of 25mg BID. At the start and end of the study, we will obtain cardiac MRI, 6 minute walk distance, brain natriuretic peptide (serum), echocardiogram, and functional class assessment. Our primary outcome is a change in right ventricular ejection fraction with cardiac MRI.