To evaluate the feasibility of using radiosensitivity of peripheral leukocytes as a predictor of clinical therapeutic responses to radiosurgery in individuals with cerebral arteriovenous malformation (AVM), we enrolled 18 patients years after they had received Gamma Knife radiosurgery for their cerebral AVM. The AVMs were shown with different degrees of regression in size in posttherapeutic periods. The peripheral leukocytes of these patients were collected at the last neuroimaging follow-ups. The leukocytes, before and 1 and 2 h after 8 Gy external gamma-irradiation, were evaluated for the amounts of DNA double-strand breaks (DSB) in 50 randomly selected individual nuclei by the neutral single cell gel electrophoresis, or so-called comet analysis. After being adjusted for gender and age at radiosurgery, the individuals with less posttherapeutic regression in AMV sizes or relatively poor or inadequate responses to radiosurgery were shown to have significantly higher DSB repair capacity on their leukocytes by comet analysis. These results suggested that in vitro radiosensitivity of peripheral leukocytes may provide valuable information for predicting therapeutic response or for adjusting irradiation doses in AVM radiosurgery.