Primary leptomeningeal gliomatosis is rare, and the diffuse form is even rarer with only three cases reported in the literature. A fourth case is described in this report. Computerized tomography (CT) findings showed hydrocephalus with enhancement of the cerebral cisterns, and analysis of cerebrospinal fluid obtained by lumbar puncture showed many atypical cells. Based on these findings, a diagnosis of leptomeningeal tumor was made. There was some improvement in neurological and CT findings following radiotherapy and chemotherapy.