Thirty-five patients affected with sporadic motor neuron disease (MND) and without clinically evident mental deterioration were systematically investigated by means of neuropsychological tests, quantitative analysis of EEG and brain CT. The MND patients as a group showed a slight but definite and stereotyped cognitive impairment. Temporal slow EEG activity was increased in the whole MND group and posterior background activity was slower in the more cognitively impaired patients. No significant differences were found in CT measurements of brain atrophy between MND and controls.