In brain, breast, and other common human tumors there is a correlation between expression of the transcriptional activator hypoxia-inducible factor 1 (HIF-1) and tumor grade and vascularization. HIF-1 stimulates angiogenesis by activating transcription of the gene encoding vascular endothelial growth factor (VEGF). HIF-1 is a heterodimer consisting of a constitutively-expressed HIF-1beta subunit and an O2- and growth factor-regulated HIF-1alpha subunit. Recent studies have demonstrated that HIF-1alpha expression is increased in tumor relative to normal tissue by two mechanisms. First, decreased intratumoral O2 concentrations provide a physiological stimulus. Second, genetic alterations that activate oncogene products or inactivate tumor suppressor gene products increase HIF- 1alpha expression and/or HIF-1 transcriptional activity independent of the O2 concentration. Taken together, these recent data suggest that increased HIF-1 activity provides a molecular basis for VEGF-induced angiogenesis and other adaptations of cancer cells to hypoxia that are critical for establishment of a primary tumor and its progression to the lethal phenotype.