Diffuse astrocytomas of the cerebrum, cerebellum, brain stem, and spinal cord are classified into three groups according to the degree of tumor anaplasia. These groups are the astrocytoma, anaplastic astrocytoma, and glioblastoma multiforme. Juvenile pilocytic astrocytomas have a better prognosis and are clinically and biologically distinct from the diffuse, fibrillary astrocytomas. The prognosis of astrocytomas depends not only on histologic characteristics, but also age of the patient, location of the tumor, and extent of surgical resection. The pattern of invasion into surrounding brain distinguishes gliomas from metastatic carcinomas and sarcomas. Topographic correlations have shown that malignant gliomas may invade the brain for distances of up to several centimeters from the enhancing rim seen on CT scan. However, the junction between glioblastoma and adjacent brain may also be fairly abrupt, with a peripheral margin of less than 1 mm. Recurrent glioblastomas are more widely invasive and often extend into areas that appear normal on CT scan. The optimal site for tumor biopsy corresponds to areas of contrast enhancement. Primitive neuroepithelial tumors are malignant neoplasms with a poor prognosis. They tend to recur locally and metastasize throughout the neuraxis via the CSF. It remains controversial whether these tumors should be classified as a single entity with the potential for differentiation along different cell lines, or whether the categories of neuroblastoma, spongioblastoma, ependymoblastoma, pineoblastoma, and medulloblastoma should be retained as specific entities. The medulloblastoma is the most common of these neoplasms, its clinicopathologic features are well characterized, and the current 5-year survivals of 50 to 60 per cent are better than for other "primitive" neoplasms. Glial fibrillary acidic protein is a specific marker for immature, reactive, and neoplastic astrocytes and ependymal cells. Although the absence of GFAP in a neoplasm does not exclude an astrocytic origin, the presence of GFAP indicates astrocytic or ependymal differentiation. This has important diagnostic applications. The expression of GFAP is used to distinguish astrocytic neoplasms from epithelial or mesenchymal tumors that may on occasion mimic a glioma. The detection of GFAP is also useful in the investigation of tumor histogenesis and differentiation both in vivo and in vitro. Although meningiomas exhibit a wide variety of histologic patterns, most tumors exhibit similar biologic and clinical behavior regardless of the histologic subtype.(ABSTRACT TRUNCATED AT 400 WORDS)