The case is described of a man, aged 46 at his time of death, who suffered from focal motor, adversive and generalised seizures for 22 years. He developed a progressive dementia over the last 2 years of his life. Investigation, including angiography and air encephalography early in the course of the illness and repeated CT head scans later, failed to demonstrate any neoplasm. Death occurred abruptly, due to cerebellar tonsillar herniation. At postmortem an extensive diffuse low grade fibrillary astrocytoma infiltrated both cerebral hemispheres, the corpus callosum, central grey matter, midbrain and pons. Thus, there was gliomatosis cerebri. Attention is drawn to the exceptional length of the history in this case, the difficulties which may arise in displaying diffusely infiltrating low grade astrocytomas radiologically and to the rare occurrence of gliomatosis cerebri.