Aging is associated with impaired angiogenesis (new blood vessels formation from the endothelial cells of pre-existing vessels) in a variety of tissues. The precise mechanisms of aging-related impairment of angiogenesis are not known. PTEN is a dual-specificity phosphatase that antagonizes in some cells the PI3K/Akt signaling pathway, important for cell survival, function and angiogenesis. PTEN's role in aging-related impairment of angiogenesis is not known. In this study, we investigated whether expression of PTEN in endothelial cells may play a mechanistic role in aging-related impairment of angiogenesis. We demonstrated that human microvascular endothelial cells (HMVEC) derived from aging individuals (Aged-HMVEC) have: (1) significantly increased PTEN mRNA and protein levels and (2) impaired in vitro angiogenesis vs. neonatal derived HMVEC (Neo-HMVEC), and that (3) downregulation of PTEN using specific siRNA restores angiogenesis in Aged-HMVEC to normal. This is the first demonstration of increased PTEN expression in human microvascular endothelial cells derived from aging tissues and that elevated PTEN is a major factor responsible for aging-related impairment of in vitro angiogenesis. Published by Elsevier Inc.