To assess the value of EEG in distinguishing between cortical and subcortical dementia, we compared the EEGs of 22 patients with SRO syndrome with those of 22 patients who had a comparable degree of dementia due to cortical atrophy. EEG records from patients with SRO syndrome were of low voltage, but with preserved alpha rhythm at 8-10 Hz and were within normal limits in 12/22. In 8 there was an abnormal excess of theta activity and one had additional lateralizing asymmetries. One had frontal intermittent rhythmic slow wave activity. Comparison with records of 22 patients, with cortical atrophy due to Alzheimer's disease but with only mild to moderate dementia, showed a similar low frequency of non-specific abnormalities.