PTEN is a dual-specificity phosphatase that has been shown to inhibit vascular smooth muscle cell (VSMC) proliferation and migration, two key events in the ethiopathogenesis of atherosclerosis. Adenovirus-mediated PTEN overexpression inhibited the formation of vascular obstructive lesions induced by mechanical injury of the vessel wall. In this study, we investigated whether PTEN protects against atheroma formation in apolipoprotein E-null mice (apoE-/-), a widely used animal model characterized by the development of hypercholesterolemia and atherosclerosis. We examined atheroma development in the aorta of apoE-/- mice with an intact Pten gene and apoE-/- mice lacking one allele of Pten (Pten+/-apoE-/-) that were challenged for six weeks with an atherogenic diet. Compared with apoE-/- controls, Western blot analysis of arterial cell lysates from Pten+/-apoE-/- mice revealed a decrease in PTEN expression. This correlated with increased phosphorylation of AKT, thus demonstrating that Pten inactivation in Pten+/-apoE-/- mice has functional consequences. However, the extent of atherosclerosis was undistinguishable in both groups of fat-fed mice. Likewise, the atheroma of Pten+/-apoE-/- and apoE-/- mice displayed similar VSMC content, cellularity and rates of proliferation and apoptosis. Thus, in spite of the cytostatic and antimigratory activities of PTEN, and in contrast to previous studies demonstrating that Pten is haplo-insufficient for tumor suppression, our results demonstrate that atherosclerosis in hypercholesterolemic mice is not aggravated by partial inactivation of Pten.