Both stereotactic radiosurgery and microsurgery are treatment modalities for arteriovenous malformations (AVM), and more recently, multimodality treatment using these approaches has been utilized. We surgically resected AVMs from 33 patients (ages 7-64 years old, mean age 30.4) 1-11 years after radiosurgery. AVM volumes were 0.8-117 cm3 (mean 21.6 cm3), and doses ranged from 4.6-45 GyE (mean 21.2 GyE). AVMs resected were submitted for pathologic review. Each AVM was evaluated for the following radiation changes, and the number of AVMs demonstrating these changes were noted: endothelial proliferation (27), hyaline (18) and calcium (10) in AVM vessel walls, partial (9) or complete (24) thrombosis of some AVM vessels, and necrosis of vessels (15) and adjacent brain tissue (11). A semiquantitative scale (mild, moderate, severe) incorporating the aforementioned changes present in each case classified the extent of radiation-induced change. There was a significant correlation (r = 0.624, p < 0.01) between extent of radiation change and dose of radiation received. There was no absolute radiation dose threshold below which radiation-induced changes were absent. However, all but one patient receiving greater than 20 GyE developed moderate to severe radiation vascular changes and the 3 patients treated with greater than 30 GyE all had severe radiation-induced changes. Radiation changes in AVMs following stereotactic radiosurgery appear to be dose-related. The correlation of dose to extent of radiation change may allow the determination of the optimal dose of radiation to treat AVMs.