Pulmonary arterial hypertension (PAH) is a devastating disease characterised by increased pulmonary arterial pressures due to clustered proliferation of cells within the vessel. Untreated this leads to right heart failure and premature death. PAH predominates in females implicating sex hormones, in particular estrogenic metabolites, as being key in the progression of PAH phenotypes. We hypothesised that hydroxy and methoxy estrogens, with known proliferative and anti-proliferative properties, may be involved in endothelial dysfunction in PAH. This may be mediated by dysfunctional signalling via the aryl hydrocarbon receptor (AhR) which modifies cytochrome P4501A1 and 1B1 expression (enzymes generating estrogenic metabolites). An assay to measure estradiol metabolites, as their methylpiperazine derivatives in plasma was developed using liquid chromatography tandem mass spectrometry. E2 metabolites were recovered from plasma using solid phase extraction, permitting limits of detection of 4.3 pg on column. Screening of plasma (with Ethical approval) detected 16-alpha-hydroxyestradiol (16αOHE2) and 2-methoxyestrone (2MeOE1) in PAH patients but not controls; n=3 controls, n=7 patients. The effects of 16αOHE2 in the pulmonary circulation were unknown. Proliferation and migration of human control and PAH-derived blood outgrowth endothelial cells (BOECs) were assessed in the presence of 16αOHE2. Redox-dependent signalling was studied using bardoxolone, a nuclear factor erythroid-2-related factor-2 (Nrf-2) activator. 16αOHE2 increased migration of PAH-BOECs which was attenuated by bardoxolone. 16αOHE2 increased proliferation of PAH-BOECs in an AhR/Nrf-2-dependent manner. 16αOHE2 decreased mRNA levels of AhR in BOECs to a greater degree cells from PAH patients compared to controls. Metabolite profiling by LC-MS/MS of plasma from patients with PAH identified 16αOHE2 as a novel biomarker of disease. The actions of 16αOHE2 in a primary cell model suggests this metabolite may influence redox-sensitive proliferation and migration of endothelial cells in PAH.