Objective:Gamma Knife radiosurgery(GKS) has been used for treatment of metastatic brain tumors. Radiation necrosis is the most significant complication of GKS. Authors retrospectively reviewed the treatment parameters of GKS to determine if there were significant factors affecting the occurrence of radiation necrosis after GKS. Materials:From 1992 to 2002, 171 GKS procedures for 572 metastatic brain lesions in 150 patients were included in this study, Total 10 lesions in 10 patients were diagnosed as radiation necrosis. Their treatment parameters were analysed and compared with control group. Results:Of 10 patients with radiation necrosis after GKS, the primary sites were lung(7 patients), gastrointestinal tract(2 patients) and kidney(1 patient). Five patients had multiple brain lesions and the mean number of lesions were 2.6. Mean dose, maximal dose, integral dose, prescription isodose volume(PIV)/tumor volume(TV) and 10% normal brain volume showed significant difference between radiation necrosis group and non-radiation necrosis group. Patient`s age, marginal dose, prescription isodose line, number of isocenter, marginal dose, PIV and tumor volume were not related to the occurrence of radiation necrosis. Conclusion:For the treatment of metastatic brain tumors, radiosurgery has the advantage of whole brain radiation therapy. Because the radiation necrosis, which is the only serious complication of radiosurgery, is reversible condition, the goal of tumor control should not be affected by concern of this complication.