An epidermoid cyst of the 3rd ventricle was verified by necropsy in a man of 60 with three episodes of Mollaret's aseptic leukocytic-endothelial meningitis. The authors accept the interpretation as macrophages of the "endothelial" cells. They bring forward evidence for the blastomatous nature of partly cornified epithelial cells present in their case. They suggest that cholesterin containing debris entering the subarachnoid space from the tumour cysts provoke an aseptic "chemical" meningitis of short course.