Serum was collected with ethical approval from Vanderbilt Medical Center, USA for blinded analysis between clinical phenotypes; heritable and idiopathic PAH patients (hPAH n=13, 14; iPAH n=7, 12) compared to non-PAH control (n =17, 17) (female, males, respectively). Quantification of circulating estrogens in human bio-fluids may reveal insights into disease aetiology but has been hampered through inadequate sensitivity and specificity of immunoassays. Therefore, we developed and applied a liquid chromatography tandem mass spectrometry (LCMS/MS) method to profile multiple estrogen metabolites in PAH. Results are shown as mean±S.E.M. with P-values following KruskalWallis statistical tests. E2 (23.3 ± 2.5 vs. 15.1 ± 1.5 pg/ml, P=0.02) and 16OHE1 (87.3 ± 20.4 vs. 25.8 ± 1.9 pg/ml, P=0.004) were elevated in male iPAH patients vs. controls with a trend also shown toward E1 elevation (41.9 ± 8.0 vs. 24.8 ± 2.4 pg/ml, P=0.08). However, in females, only elevations of 16OHE2 were detected (23.6 ± 4.7 vs. 12.2 ± 2.2 pg/ml, P=0.005). The estrogen profiles of hPAH patients did not differ from those of non-PAH individuals. Therefore, profiling estrogen metabolism by LCMS/MS reveals increased circulating levels of specific bioactive estrogens in iPAH but not in hPAH. The exact pattern differed between male and female iPAH, in both cases the pathway of 16 hydroxylation was upregulated generating proliferative metabolites that may underpin the pathobiology of disease.