Rationale: Circulating microparticles (MPs) are submicron membrane fragments shed from damaged or activated vascular cells. Endothelial MPs are a biological marker of dysfunctional endothelium. Vascular remodeling and endothelial dysfunction are involved in pulmonary hypertension (PH). Objectives: We tested the hypothesis that circulating MPs are increased in patients with PH and that identifiable subgroups of MPs predict the hemodynamic severity of this condition progression. Methods: N=24 patients (age =54± 4 years) undergoing right heart catheterization for pre-capillary PH without any endothelium-active vasodilator therapy participated in the study. Age and gender-matched healthy controls (n=20) were included. Endothelial (PECAM+ [CD31⁺-CD41^-], VE-Cadherin+ [CD144⁺] , and E-Selectin+ [CD62e⁺]), platelet (CD41⁺), leukocyte-derived (CD45⁺) and Annexin V⁺ MPs were measured using flow cytometry in platelet-free plasma from venous blood. Results: Levels of circulating endothelial PECAM+, VE-Cadherin+, E-Selectin+, and leukocytes-derived MPs, but not platelet and AnnexinV⁺ MPs, were increased in PH subjects compared to controls (p<0.01 each). PECAM+ and VE-Cadherin+ MP levels significantly correlated with mean pulmonary artery pressure (r=0.92 and r=0.87, respectively), pulmonary vascular resistance (r=0.78 and r=0.73), mean right atrial pressure (r=0.43, and r=0.46) and correlated inversely with cardiac index (r= -0.59 and r= -0.52). These relationships were not observed for other MPs subgroups, and persisted in multivariate analysis after adjustment for confounding factors. Conclusions: In subjects with pre-capillary PH, levels of circulating endothelial and leukocytes MPs were increased compared to control subjects. In addition, levels of PECAM+ and VE-Cadherin+ but not E-Selectin+ endothelial MPs predicted hemodynamic severity of the disease.