Hypoxia is a characteristic feature of locally advanced solid tumors resulting from an imbalance between oxygen (O(2)) supply and consumption. Major causative factors of tumor hypoxia are abnormal structure and function of the microvessels supplying the tumor, increased diffusion distances between the nutritive blood vessels and the tumor cells, and reduced O(2) transport capacity of the blood due to the presence of disease- or treatment-related anemia. Tumor hypoxia is a therapeutic concern since it can reduce the effectiveness of radiotherapy, some O(2)-dependent cytotoxic agents, and photodynamic therapy. Tumor hypoxia can also negatively impact therapeutic outcome by inducing changes in the proteome and genome of neoplastic cells that further survival and malignant progression by enabling the cells to overcome nutritive deprivation or to escape their hostile environment. The selection and clonal expansion of these favorably altered cells further aggravate tumor hypoxia and support a vicious circle of increasing hypoxia and malignant progression while concurrently promoting the development of more treatment-resistant disease. This pattern of malignant progression, coupled with the demonstration of a relationship between falling hemoglobin level and worsening tumor oxygenation, highlights the need for effective treatment of anemia as one approach for correcting anemic hypoxia in tumors, and in so doing, possibly improving therapeutic response.