Radiosurgery (delivery of a high dose of radiation to a small volume of the brain) is usually well tolerated since the volume of brain irradiated is small. Despite growing interest in radiosurgery, the influence of the volume of brain irradiated on the tolerance of the brain to radiation is not well understood. The results of six studies reporting clinically significant radiation reactions following radiosurgery for arteriovenous malformations are reviewed. In the combined series, 23 (9%) of 255 patients developed a clinically significant radiation reaction. The volume of brain irradiated and the dose delivered in each of these 23 patients are presented. The compiled data suggest that the previously presented guidelines may underestimate the risks of radiosurgery. Additional clinical and experimental data are needed to determine "safe" doses of radiation to be used during radiosurgery.