CD44 membrane glycoprotein is involved in angiogenesis, but it is not clear whether CD44 functions as a pro- or anti-angiogenic molecule. Here, we assess the role of CD44 in angiogenesis and endothelial proliferation by using Cd44-null mice and silencing in human endothelial cells. We demonstrate that angiogenesis is increased in Cd44-null mice compared to either wild type or heterozygous animals. Silencing of CD44 expression in cultured endothelial cells results in augmented proliferation and viability. CD44's growth-suppressive effect is mediated by its extracellular domain and it is independent of its hyaluronan binding function. CD44-mediated effect on cell proliferation is independent of specific angiogenic growth factor stimulation. These results show that CD44 expression on endothelial cells constrains endothelial cell proliferation and angiogenesis. Thus, endothelial CD44 might serve as a therapeutic target in either cardiovascular diseases where endothelial protection is desired or, in contrast, as an antiangiogenic molecule for cancer treatment.