There is evidence that pentoxifylline may be both a radioprotector of normal tissue and a radiosensitizer of tumor cells. This article reviews this evidence and then describes our own laboratory study to determine whether pentoxifylline is a radiosensitizer of human glioblastoma cells in vitro. Human glioblastoma multiforme cells (SNB19 cell line) were irradiated in vitro with and without pentoxifylline. Regression of the log ratios (quotient of surviving colonies) revealed greater tumor cell kill in the PTX group, and the difference increased as the radiation dose increased (p < 0.01 at the 750 and 1000 cGY doses). Before the hypothesis that PTX is a radiosensitizer of hypoxic tumor cells can be confirmed or denied, it must be determined if the agent also has a separate mechanism of tumoricidal activity. Whereas in vivo models allow the well-documented rheologic, immunologic and oxygen-related effects of PTX to be active simultaneously, the in vitro model described herein excludes the effects of such systemic actions and focuses on mechanisms at the cellular or subcellular level. These data suggest there exists such a mechanism of tumoricidal activity of PTX that has not been previously identified.