Glioblastoma U251 and U87 cells irradiated with single fraction high dose rate radiation (1.1 Gy/min) were relatively insensitive to inactivation of colony forming ability, similar to other glioblastoma cell lines. Initial rates of cell kill with continuous low dose rate irradiation (0.075 Gy/hr to 0.49 Gy/hr) were low, but at times greater than 20 hours and with dose rates of 0.25 Gy/hr or higher, the rate of cell kill increased. Population doubling times for these cell lines were about 24 hours, suggesting that cell cycle redistribution may be responsible for the increased sensitivity. DNA histograms obtained by flow cytometry support this hypothesis, with cells accumulating in the G2 and M phases of the cell cycle. These results suggest that low dose rate irradiation may be effective in treating glioblastomas. Optimization of time intervals between radiation treatments as well as dose rates used for glioblastoma patients may be influenced by these findings, resulting in better integration of continuous low-dose-rate irradiation (radioactive antibodies and implants) and high-dose-rate irradiation (fractionated external beam) into therapeutic programs.