Purpose : This study was peformed to determine the optimal radiation therapy field for the treatment of malignant astrocytoma and glioblastoma multiforme. Materials and Methods : From Jan. 1994 to Mar. 2000, 21 patients with malignant astrocytoma and glioblastoma multiforme, confirmed as recurrent by follow up MRI after surgery and radiation therapy, were analyzed. The distance from the margin of the primary lesion to the recurrent lesion was measured. The following factors were analyzed to Investigate the influence of these factors to recurrence pattern; tumor size, degree of edema, surgical extent, gamma knife radiosurgery and multiple lesions. Results : Among the 21 patients, 18 $(86\%)$ were recurred within 2 cm from the primary lesion site. 12 within 1 cm, 6 between 1 and 2 cm. The other 3 patients all with multiple lesions, were recurred at 3, 4, 5 cm, from the primary lesion site. The recurrence pattern was not influenced by the factors of tumor size, extent of edema, surgical extent, or gamma knife radiosurgery. However, patients with multiple lesions showed a tendency of recurrence at sites further from the primary lesion. Conclusions : Most $(86\%)$ of the recurrences of malignant astrocytoma and glioblastoma multiforme occurred within 2 cm from the primary lesion site. The width of treatment field does not need to be changed according to tumor size, degree of edema, surgical extent, or gamma knife radiosurgery. However, the treatment field for multiple lesions appears to be wider than that for a single lesion.