The existence of cells circulating in the peripheral blood with the capacity of an endothelial progenitor cell (EPC) remains controversial. This is best exemplified by the original cell posited by Asahara and colleagues to be an EPC in 1997,1 which has now been clearly phenotyped as a monocyte with dendritic features.2 There remains a viable candidate known as the endothelial colony-forming cell (ECFC) or endothelial outgrowth cell (OEC), named because it cannot be seen in ex vivo culture for at least 10 days. These cells proliferate in culture, form a cobblestone monocellular layer, and form networks in ex vivo assays. Given the lack of understanding of previous candidate cells we sought to better characterise the EOC. Here we present data from electron microscopy studies, immunohistochemistry, ligand stimulation studies, and mRNA microarrays that demonstrate the EOC is a true endothelial cell. Furthermore we demonstrate that these cells can potentially be used as endothelial surrogates in patients with pulmonary hypertension due to a mutation in the bone morphogenetic protein type II receptor (BMPRII). Cells taken from the peripheral blood of patients have a deficiency in intracellular signalling in the BMPRII pathway on stimulation with BMP9. This can be demonstrated by reduced phosphoSmad 1/5 protein on western blotting, and downstream with reduced Id1 gene induction by qPCR. Therefore regardless of its origin and biologically intended function, the EOC is an endothelial cell, and has the capacity to act as an easily derivable endothelial surrogate from patients in whom vascular tissue is not normally obtainable.