The tumor oxygenation status associates with aggressive behavior. Oxygen shortage, hypoxia, is a major driving force behind tumor vascularization, and hypoxia enhances mutational rate, metastatic spread, and resistance to radiation and chemotherapy. We recently discovered that hypoxia promotes dedifferentiation of neuroblastoma and breast carcinoma cells and development of stem cell-like features. In both these tumor forms there is a correlation between low differentiation stage and poor outcome, and we conclude that the dedifferentiating effect of lowered oxygen adds to the aggressive phenotype induced by hypoxia. With neuroblastoma and breast carcinoma as human tumor model systems, we have addressed questions related to hypoxia-induced molecular mechanisms governing malignant behavior of tumor cells, with emphasis on differentiation and growth control. By global gene expression analyses we are currently screening for gene products exclusively expressed or modified in hypoxic cells with the aim to use them as targets for treatment.