To define tumor boundaries prior to radiation therapy, 13 patients with intracranial glial neoplasms had tumor biopsies using magnetic resonance imaging (MRI)--computed tomography (CT) stereotactic technique. Precise histologic sampling of the lesion volume was correlated with the MRI and CT-defined 'lesion'. Tumor boundaries extended beyond the CT or MRI margin in 4 of 4 cases of glioblastoma, 1 of 3 cases of anaplastic astrocytoma, and 1 of 6 cases of well-differentiated astrocytoma. While stereotactic MRI is superior to stereotactic CT in the visualization of glial neoplasms, accurate tumor biopsy is mandatory to define both tumor type and margins before pre-operative radiation therapy.