The vascular endothelial growth factors (VEGF) plays major roles during tumor development. VEGF expression is regulated by the transcription factor HIF (hypoxia inducible factor) which is very sensitive to oxygene variations. We will show here that HIF1alpha is today a good prognostic and diagnostic marker for an increasing number of cancers from various origins. Furthermore, the reduction of the expression or the activity of HIF1alpha impairs tumour angiogenesis and subsequently growth. Altogether, these results suggest that HIF1alpha could be an interesting target in cancer therapy.