A 28 year old woman suffered from lumbar radicular signs and sensory deficit for three years before death. A progressive illness with increased intracranial pressure, cerebellar signs, and multiple cranial nerve palsies appeared one year after the onset of symptoms, but craniotomy of the posterior fossa was negative. After development of spastic paraparesis, an intramedullary oligodendroglioma of the lower thoracic cord was partly removed. Death occurred two months later from raised intracranial pressure. At autopsy a diffuse oligodendroglioma was found involving the periventricular regions of the brain, the entire subarachnoid space, the cerebral parenchyma, and the whole length of the thoracic cord. There was a small lumbar syrinx. It is suggested in this case that the dorsal intramedullary oligodendroglioma was the source from which tumour spread diffusely through the cerebrospinal fluid pathways and secondarily invaded the cerebrum. The diagnostic difficulties of diffuse meningeal gliomatosis are discussed.