C arcinoembryonic antigen–related cell adhesion molecule 1 (CEACAM1), a cellular adhesion molecule of the Ig superfamily, is associated with early stages of angiogenesis. In vitro, CEACAM1 regulates proliferation, migration, and differentiation of murine endothelial cells. To prove that CEACAM1 is functionally involved in the regulation of vascular remodeling in vivo, we analyzed 2 different genetic models: in Ceacam1^–/– mice, the Ceacam1 gene was deleted systemically, and in CEACAM1^endo+ mice, CEACAM1 was overexpressed under the control of the endothelial cell–specific promoter of the Tie2 receptor tyrosine kinase. In Matrigel plug assays, Ceacam1^–/– mice failed to establish new capillaries whereas in CEACAM1^endo+ mice the implants were vascularized extensively. After induction of hind limb ischemia by femoral artery ligation, Ceacam1^–/– mice showed significantly reduced growth of arterioles and collateral blood flow compared with their WT littermates. In agreement with a causal role of CEACAM1 in vascular remodeling, CEACAM1^endo+ mice exhibited an increase in revascularization and collateral blood flow after arterial occlusion. Our findings indicate that CEACAM1 expression is important for the establishment of newly formed vessels in vivo. Hence CEACAM1 could be a future target for therapeutic manipulation of angiogenesis in disease.