Primary diffuse leptomeningeal gliomatosis (PDLG) is a rare, fatal, neoplastic condition of infiltrating glial cells into the meninges without evidence of primary tumor in the brain or spinal cord parenchyma. Primary diffuse leptomeningeal gliomatosis often presents with symptoms and physical findings of chronic inflammatory meningitis and raised intracranial pressure, and lacks specific clinical, radiologic, and diagnostic criteria. We report a case of PDLG diagnosed post-mortem, highlighting the diagnostic difficulty in identifying PDLG as the cause of chronic meningitis, even when a neoplastic etiology is suspected. Because multiple cytologies and even a leptomeningeal biopsy did not reveal the diagnosis ante-mortem, we emphasize the consideration of multi-site or repeat leptomeningeal biopsy when a persistent inflammatory infiltrate is found and neurological symptoms are progressive.