Angiogenesis is a key step in organ development and remodelling during embryogenesis or tissue regeneration. Some pathological events such as tumor growth or diabetic retinopathy also lead to angiogenesis formation. Several molecules have already been identified as promoting angiogenesis in vivo. We have recently purified a new angiogenic growth factor. Its unique specificity for vascular endothelial cells led us to provisionally name it vasculotropin (VAS) or vascular endothelial growth factor (VEGF) or vascular permeability factor (VPF). In vitro, despite a moderate action on proliferation, VAS/VEGF strongly stimulates cell migration. In vivo, VAS/VEGF is a potent inducer of angiogenesis and vascular permeability. Its central role in angiogenesis is emphasized by the observation that its immunoneutralization prevents tumor progression.