18-FDG and 11C methionine PET scans were performed on two patients with gliomatosis cerebri. The cortical grey matter was hypometabolic when compared with normal. The findings support the concept that the cerebral cortex becomes functionally disconnected in this disease owing to the infiltrative nature of the underlying tumour. This may account for the high incidence ofdementia in the course ofthis disease. In one of the cases described here, there wasclear evidence of progression from a discrete tumour mass of glioma to gliomatosis cerebri and this progression argues against the WHO classification of this disorder separately from glioma.