Rational Transplantation of human CD34+ stem cells to ischemic tissues has been associated with reduced angina, improved exercise time and reduced amputation rates in phase 2 clinical trials and has been shown to induce neo-vascularization in pre-clinical models. Previous studies have suggested that paracrine factors secreted by these pro-angiogenic cells are responsible, at least in part, for the angiogenic effects induced by CD34+ cell transplantation. Objective Our objective was to investigate the mechanism of CD34+ stem cell induced pro-angiogenic paracrine effects and to examine if exosomes, a component of paracrine secretion, are involved. Methods and Results Exosomes collected from the conditioned media of mobilized human CD34+ cells had the characteristic size (40–90 nm; determined via dynamic light scattering), cup-shaped morphology (electron microscopy), expressed exosome-marker proteins CD63, phosphatidylserine (flow cytometry) and TSG101 (immunoblotting), besides expressing CD34+ cell lineage marker protein, CD34. In vitro, CD34+ exosomes replicated the angiogenic activity of CD34+ cells by increasing endothelial cell viability, proliferation and tube formation on Matrigel. In vivo, the CD34+ exosomes stimulated angiogenesis in Matrigel plug and corneal assays. Interestingly, exosomes from CD34+ cells, but not from CD34+ cell-depleted mononuclear cells had angiogenic activity. Conclusions Our data demonstrate that human CD34+ cells secrete exosomes that have independent angiogenic activity both in vitro and in vivo. CD34+ exosomes may represent a significant component of the paracrine effect of progenitor-cell transplantation for therapeutic angiogenesis.