Diffuse or multifocal invasion of the leptomeninges by malignant glioma (meningeal gliomatosis) is believed to be rare. From 1971 through 1977, 11 of 52 patients with intracranial malignant gliomas examined at autopsy were found to have meningeal gliomatosis, and 1 additional patient was diagnosed clinically without autopsy (12 cases total). Eight of the 12 patients were diagnosed antemortem with positive cerebrospinal fluid (CSF) cytology, while the other 4 patients were diagnosed at autopsy only. All 11 autopsied patients had multifocal or diffuse meningeal tumor distant from the primary site; 8 patients had spinal subarachnoid seeding with tumor encroachment of cauda equina and spinal nerve roots, and 9 patients had tumor invasion into the lateral ventricles. Three patients had symptomatic spinal cord compression at the thoracic or lumbar level, and 10 patients had hydrocephalus. These 12 patients with meningeal gliomatosis were compared with the other 41 autopsied malignant glioma patients without the complication (controls); the patients with meningeal gliomatosis were significantly younger (mean age, 40 versus 57 years; p less than 0.005). Patients with meningeal gliomatosis lived somewhat longer (median, 49 weeks) compared to controls (35 weeks), but the difference was not statistically significant. With the advance of chemotherapy, patients with malignant glioma are living longer and the incidence of meningeal gliomatosis may rise. The diagnosis of meningeal gliomatosis can be suspected, especially if hydrocephalus is present, and can often be confirmed by CSF cytology.