Aims: Solid tumors commonly evolve hypoxic regions resulting from a low rate of blood vessel formation compared to the rate of tumor cell proliferation. These hypoxic tumors are poorly sensitive towards radiation therapy due to an upregulation of the transcription factor HIF (hypoxia-inducible factor). The aim of this study therefore was to investigate if either RNAi treatment or incubation with the natural compound curcumin could impair tumor cells in vitro by targeting HIF-a. Methods: Several cells lines originated from different human tissues were either transfected with si- or shRNA to inhibit HIF-a expression or were pretreated with curcumin. Before irradiation cells were cultured under well defined oxygen concentrations and radiation effects were investigated by clonogenic assays. Impact of curcumin on HIF protein levels and activity was estimated by immunoblotting and reporter gene assays. Results: Downregulation of HIF-a by either RNAi or by curcumin resulted in an increased radiation sensitivity of several tumor cells in vitro. Curcumin reduced HIF-1 levels and activity by degrading ARNT and exhibited a radiosensitizing and antiproliferative effect on hepatoma cells. RNAi experiments demonstrated HIF-deficient cells to be less radioresistent compared to control cells under normoxic and hypoxic conditions. Conclusion: Curcumin as well as other HIF-a inhibiting agents are promising tools to improve radiotherapy of hypoxic tumors normally correlated with a poor outcome of the disease. Natural compounds as curcumin, exhibiting a low toxicity in human tissue, are shown to have therapeutic potential in adjuvant therapy of cancer evidenced by a number of ongoing clinical trials.