Estrogen Receptor Antagonist in Patients With Pulmonary Arterial Hypertension

Inclusion Criteria: * Previous documentation of mean pulmonary artery pressure \> 25 mm Hg with a pulmonary capillary wedge pressure (or left ventricular end-diastolic pressure) \< 16 mm Hg and pulmonary vascular resistance \> 3 WU at any time before study entry. * Diagnosis of PAH which is idiopathic, heritable, drug- or toxin-induced, or associated with connective tissue disease, congenital heart disease, portal hypertension, or HIV infection. * Most recent pulmonary function tests with FEV1/FVC \>50% AND either a) total lung capacity \> 70% predicted or b) total lung capacity between 60% and 70% predicted with no more than mild interstitial lung disease on computerized tomography scan of the chest. * Female, post-menopausal state, defined as: * \> 50 years old and a) have not menstruated during the preceding 12 months or b) have follicle-stimulating hormone (FSH) levels \> 40 IU/L or * \< 50 years and FSH \> 40 IU/L or * having had a bilateral oophorectomy. * Informed consent. Exclusion Criteria: * Age \< 18. * Treatment with estrogen or anti-hormone therapy (tamoxifen, anastrozole, etc.) * WHO Class IV functional status. * History of breast cancer. * Clinically significant untreated sleep apnea. * Left-sided valvular disease (more than moderate mitral valve stenosis or insufficiency or aortic stenosis or insufficiency), pulmonary artery or valve stenosis, or ejection fraction \< 45% on echocardiography. * Initiation of PAH therapy (prostacyclin analogues or receptor agonists, endothelin-1 receptor antagonists, phosphodiesterase-5 inhibitors, soluble guanylate cyclase stimulators) within three months of enrollment; the dose must be stable for at least 3 months prior to Baseline Visit. PAH therapy which is stopped and then restarted or has dose changes which are not related to initiation and uptitration will be allowed within 3 months prior to the Baseline Visit. * Hormone therapy. * Use of warfarin or other anticoagulant (use of aspirin is permitted). * Platelet count \<100,000. * Renal failure (creatinine \>/= 2.0). * Child-Pugh Class C cirrhosis. * Current or recent (\< 6 months) chronic heavy alcohol consumption. * Current use of another investigational drug (non-FDA approved) for PAH.