In neuroblastoma specimens, HIF-2α but not HIF-1α is strongly expressed in well-vascularized areas. In vitro, HIF-2α protein was stabilized at 5% O₂ (resembling end capillary oxygen conditions) and, in contrast to the low HIF-1α activity at this oxygen level, actively transcribed genes like VEGF. Under hypoxia (1% O₂), HIF-1α was transiently stabilized and primarily mediated acute responses, whereas HIF-2α protein gradually accumulated and governed prolonged hypoxic gene activation. Knockdown of HIF-2α reduced growth of neuroblastoma tumors in athymic mice. Furthermore, high HIF-2α protein levels were correlated with advanced clinical stage and high VEGF expression and predicted poor prognosis in a clinical neuroblastoma material. Our results demonstrate the relevance of HIF-2α in neuroblastoma progression and have general tumor biological implications.