The Notch ligand Dll4 has a recognized role during both physiologic and tumor angiogenesis, as it contributes to regulate Notch activity in endothelial cells (EC). The effects of Dll4 on Notch signaling in tumor cells expressing Notch receptors remain, however, largely unknown. Here, we report that escape of human T-cell acute lymphoblastic leukemia (T-ALL) cells or colorectal cancer cells from dormancy is associated with Dll4 expression in the tumor microenvironment and increased Notch3 signaling in tumor cells. Dll4 was expressed at early time points during the angiogenic process, and its expression preceded perfusion of the newly established vessels. Treatment of EC with angiogenic factors induced Dll4 expression and increased Notch3 activation in cocultured T-ALL cells. Neutralization of Dll4 greatly reduced EC-mediated activation of Notch 3 signaling in T-ALL cells and blocked tumorigenesis. Moreover, silencing Notch3 by RNA interference had marked antiproliferative and proapoptotic effects on T-ALL cells in vitro and reduced tumorigenicity in vivo. Our results elucidate a novel mechanism by which a direct interplay between endothelial and tumor cells promotes survival and triggers tumor growth.