Atypical teratoid/rhabdoid tumor is a rare, highly malignant central nervous system tumor most commonly occurring in very young children. Atypical teratoid/rhabdoid tumor most often presents as an expanding mass with symptoms consistent with the location of the tumor and may present with metastatic leptomeningeal disease. The authors describe 2 cases of rapidly progressive, diffuse leptomeningeal atypical teratoid/rhabdoid tumor without a solid primary mass. These cases demonstrate a clinical picture that can easily be confused with a basilar meningitis, encephalomyelitis, or vasculitis.