Gliomatosis cerebri (GC) describes a diffuse overgrowth of the neuraxis by neoplastic glial cells with relative preservation of the underlying cytoarchitecture. Ten cases of GC are presented, followed by discussions of its radiological and pathological characteristics. The diagnosis of GC was formerly made only at autopsy. However, recent improvements in imaging and biopsy now allow for antemortem diagnosis. Immunocytochemistry for S-100 protein and glial fibrillary acidic protein (GFAP) in three open biopsies of early primary GC revealed diffuse proliferation of S-100 immunoreactive and GFAP-resistant small astrocytes. Magnetic resonance imaging appears to be the imaging modality of choice. T2-weighted images generally show contiguous high intensity signal in affected areas. Biopsy, either open or stereotactic, remains necessary for antemortem diagnosis. No treatment has proven effective for GC, the prognosis being poor. The authors also review theories of tumorigenesis of GC in an effort to explain its biology.