Inhibitor of DNA binding 1 (Id-1) has been implicated in tumor angiogenesis by regulating the expression of vascular endothelial growth factor (VEGF), but its molecular mechanism has not been fully understood. Here, we show the cross talk between Id-1 and hypoxia-inducible factor-1α (HIF-1α), that Id-1 induces VEGF by enhancing the stability and activity of HIF-1α in human endothelial and breast cancer cells. Although both the transcript and proteins levels of VEGF were induced by Id-1, only the protein expression of HIF-1α was induced without transcriptional changes in both human umbilical endothelial cells and MCF7 breast cancer cells. Such induction of the HIF-1α protein did not require de novo protein synthesis but was dependent on the active extracellular response kinase (ERK) pathway. In addition, stability of the HIF-1α protein was enhanced in part by the reduced association of the HIF-1α protein with von Hippel-Lindau protein in the presence of Id-1. Furthermore, Id-1 enhanced nuclear translocation and the transcriptional activity of HIF-1α. Transcriptional activation of HIF-1–dependent promoters was dependent on the active ERK pathway, and the association of HIF-1α protein with cyclic AMP-responsive element binding protein was enhanced by Id-1. Finally, Id-1 induced tube formation in human umbilical endothelial cells, which also required active ERK signaling. In conclusion, we provide the molecular mechanism of the cross talk between HIF-1α and Id-1, which may play a critical role in tumor angiogenesis. (Mol Cancer Res 2007;5(4):321–9)