Tamoxifen Therapy to Treat Pulmonary Arterial Hypertension

Inclusion Criteria: * Previous documentation of mean pulmonary artery pressure greater than or equal to 25 mm Hg with a pulmonary capillary wedge pressure (or left ventricular end-diastolic pressure) less than or equal to15 mm Hg and PVR greater than or equal to 3 WU at any time before study entry, consistent Group 1 PAH classified by accepted international classification. * Diagnosis of PAH which is idiopathic, heritable, drug- or toxin-induced, or associated with connective tissue disease. * Age 18 years and older. * WHO Functional Class I, II, or III status. * Ability to perform a six minute walk test without significant limitations in musculoskeletal function or coordination, with distance greater than or equal to 150m and less than or equal to 550m. * Informed consent. Exclusion Criteria: * Current treatment with estrogen, progesterone, or any form of sex hormone therapy. * Current treatment with anti-sex hormone therapy (e.g., anastrozole, fulvestrant, tamoxifen, leuprolide acetate (luporon) or other centrally-acting hormone agents. * WHO Functional Class IV status. * History of, or current, breast, uterine, ovarian, or testicular cancer. * Current pregnancy, or prior pregnancy within 3 months of enrollment. * Initiation of PAH therapy (prostacyclin analogues, endothelin-1 receptor antagonists, phosphodiesterase-5 inhibitors, riociguat, selexipag) within three months of enrollment; the dose must be stable for at least three months prior to Baseline Visit. Of note, PAH therapy, including diuretics, which is stopped and then restarted or has dose changes which are not related to initiation and up titration will be allowed within 3 months prior to the Baseline Visit, and during the trial for subjects. * History of thromboembolic event. * Hospitalized or acutely ill. * Renal failure (creatinine over 2.0). * Hypercalcemia. * Severe osteoporosis (t score \< -2.0 OR t score \< -2.5 if on bone modifying treatment). * Current or recent (\< 3 months) chronic heavy alcohol consumption. * Enrollment in a clinical trial or concurrent use of another investigational drug (non FDA approved) or device therapy within 30 days of screening visit. * Enrollment in any pharmacologic clinical trial within one month of screening. * Due to potential drug interactions with tamoxifen, subjects using bosentan (CYP3A4) or selexipag (CYP2C8) will be excluded. * Due to the concerns of pregnancy during PAH and with tamoxifen use, subjects will be excluded who do not use at least two forms of contraception (e.g., IUD plus the use of a barrier contraceptive method).