© 2014 by the Pulmonary Vascular Research Institute. All rights reserved. The pathophysiologic alterations of patients with pulmonary arterial hypertension (PAH) are diverse. We aimed to determine novel pathogenic pathways from circulating proteins in patients with PAH. Multianalyte profiling (MAP) was used to measure 90 specifically selected antigens in the plasma of 113 PAH patients and 51 control patients. Erythropoietin (EPO) functional activity was assessed via in vitro pulmonary artery endothelial cell networking and smooth muscle cell proliferation assays. Fifty-eight patients had idiopathic PAH, whereas 55 had other forms of PAH; 5 had heritable PAH, 18 had connective tissue disease (15 with scleroderma and 3 with lupus erythematosis), 13 had portopulmonary hypertension, 6 had PAH associated with drugs or toxins, and 5 had congenital heart disease. The plasmaantigen profile of PAH revealed increased levels of several novel biomarkers, including EPO. Immune quantitative and histochemical studies revealed that EPO not only was significantly elevated in the plasma of PAH patients but also promoted pulmonary artery endothelial cell network formation and smooth muscle cell proliferation. MAP is a hypothesis-generating approach to identifying novel pathophysiologic pathways in PAH. EPO is upregulated in the circulation and lungs of patients with PAH and may affect endothelial and smooth muscle cell proliferation.