Meningeal carcinomatosis without gross tumour in the substance of the brain or spinal cord has been reported rarely. Two cases observed at the Victoria General Hospital, Halifax, presented a bizarre clinical picture consisting of signs of meningeal irritation without fever, and psychotic behaviour. Examination of the cerebrospinal fluid revealed low sugar concentration and increased pressure, protein and cells. In one case these cells were readily identified as malignant on stained smears. At autopsy the surfaces of the cerebral hemispheres, cerebellum and brain stem were covered by an opalescent film and on section the subarachnoid space was densely packed with malignant cells. Both primary tumours were adenocarcinomas, one originating in the gallbladder and one in the rectum. The diagnosis of meningeal carcinomatosis must be considered in patients presenting with profound mental changes and meningeal irritation without fever. Diagnosis may be confirmed by cytological examination of the cerebrospinal fluid. The primary tumour is most commonly an adenocarcinoma. There is no satisfactory treatment available.