Four children with 'epilepsy and continuous spike-wave sleep' syndrome and mental and behavioural regression were followed prospectively with neuropsychological tests, behavioural questionnaires and sleep EEG performed at regular intervals between 1 1/2 and four years of age. The children showed a pattern of behavioural and cognitive disturbances similar to that found in some developmental autistic-like disorders, but also in adult frontal syndrome. Deterioration was probably due to an unusual long-standing epileptic dysfunction involving the frontal lobes. The process is potentially reversible and seems to be the same as postulated for acquired epileptic aphasia, but in a different localization.