2. Patients must have histologically confirmed supratentorial grade III or IV astrocytoma (anaplastic astrocytoma, anaplastic oligodendroglioma, glioblastoma multiforme) and require a stereotactic biopsy for confirmation of tumor progression or differentiation of tumor progression from treatment induced effects following radiation therapy ± chemotherapy. Patients with previous low-grade glioma who progressed after radiotherapy ± chemotherapy and are in need of a stereotactic biopsy to confirm the presence of a high-grade glioma, and this is accomplished at the time of biopsy, are eligible.