From the Departments ofNeurology * and Clinical Neurophysiology,f University Central Hospital of Kuopio,

SUMMARY Spectral analysis of EEG was conducted for 51 elderly delirious patients meeting the Diagnostic and Statistical Manual ofMental Disorders III (DSM-III) criteria and for 19 controls. As a whole group, and also when subdivided according to the type of delirium, severity of cognitive decline or the type ofcentral nervous system disease, delirious patients showed significant reductions of alpha percentage, increased theta and delta activity and slowing of the peak and mean frequencies and these changes were also obvious in individual recordings. The alpha percentage and various ratio parameters correlated significantly with Mini Mental State score, and delta percentage and mean frequency with the lengths of delirium and hospitalisation. The results indicate an association between spectral EEG changes and severity of cognitive deterioration in delirium. Delirium is an organic mental disorder characterised by disorganisation of mental functioning due to widespread, temporary derangement of cerebral metabolism appearing symptomatically during the course of an underlying physical illness."-3 It features concurrent disorders of attention, perception, think