The switch to the angiogenic phenotype represents a critical checkpoint during tumor progression. The acquisition of new capillary vessels provides newly vascularized tumor nodules with a distinct biological advantage over their avascular counterparts by conferring upon them the ability to expand and develop both locally and metastatically. To identify the molecules and mechanisms underlying this rate-limiting step in successful tumorigenesis, we have developed an in vivo tumor model that reproducibly recapitulates the angiogenic switch. Using this model, we have analyzed vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and hypoxia-inducible factor-1alpha (HIF-1alpha) expression and activity in both avascular and vascular growth phases of the tumor. A significantly higher level of VEGF protein was detected in avascular tumor nodules compared with vascular nodules. As avascular tumors became vascularized, VEGF levels decreased approximately 10-fold. In contrast, bFGF levels were not elevated in avascular nodules but rather were detected at levels approximately 2 times higher in vascular nodules compared with the avascular tumor nodules. Given that VEGF is transcriptionally regulated by HIF-1alpha, immunohistochemical studies of chondrosarcoma nodules were conducted and revealed that the nuclear translocation of HIF-1alpha was detected exclusively in avascular tumor nodules. This study implicates HIF-1alpha-mediated up-regulation of VEGF but not bFGF in the switch to the angiogenic phenotype during tumorigenesis.