The woman presented to an emergency room with a headache and vomiting, subsequently became unresponsive, and died 14 hours later. Autopsy revealed a diffuse extensive infiltrate of well-differentiated astrocytoma in the leptomeninges of the brain and spinal cord without an underlying parenchymal tumor. Primary diffuse leptomeningeal gliomatosis is a rare tumor that arises within the leptomeninges from small neuroglial heterotopic rests that undergo neoplastic transformation. Grossly, this tumor can mimic leptomeningeal carcinomatosis, pachymeningitis, tuberculosis, sarcoidosis, and fungal infections; however, the histologic features of primary diffuse leptomeningeal gliomatosis should allow it to be readily distinguished from grossly similar conditions. The mechanism of death in this case was most likely tumor obstruction of cerebrospinal fluid outflow, resulting in the usual complications seen with increased intracranial pressure. Although this tumor is aggressive and is associated with a rapidly progressive fatal course, it had not been previously associated with sudden death. 3 figures and 18 references