Hypoxia is a common feature of many malignant neoplasms and has been identified as a major reason for the radioresistance of malignant tumor tissue. On the other hand, the transcription factor hypoxia-inducible factor-1 (HIF-1) has attracted attention because it is rapidly expressed when tissue oxygen tension is reduced, thus playing the role of a hypoxic sensor. We investigated whether the level of HIF-1 expression in glioblastoma (GBM) could be an indicator in a protocol for postoperative radiotherapy. We immunohistologically evaluated the level of HIF-1 expression in 60 pathologically diagnosed GBMs. HIF-1 expression was positive in all tumors. The progression-free survival of the group receiving only conventional radiotherapy after surgery differed significantly according to the level of HIF-1 expression. The results of this study suggest that the HIF-1 expression level can be an indicator of tumor radioresistance, and that prior knowledge of the HIF-1 expression level allows the formulation of a protocol for postoperative radiotherapy.