In the childhood epileptic encephalopathies mental impairment is common and severe. Traditional cognitive assessment is difficult because of the low level of performance, autistic features, and the unpredictable effect of seizures. An alternative is to measure adaptive and maladaptive behaviour using instruments administered to the caregivers. Adults with different types of dementia have characteristic patterns of cortical glucose hypometabolism. Thirty-two children were studied using visual and semiquantitative analysis of 18fluorodeoxyglucose positron emission tomographic (PET) scans. The Vineland Scales and the Conners' Questionnaires were used to assess adaptive and maladaptive behaviour. The mean adaptive behaviour composite score was 37.3+/-15.6; all but one subject had a low adaptive level. A profile of relative strength in socialisation and weakness in daily living skills emerged. Up to two-thirds of children had abnormal behaviour patterns, particularly attention-deficit disorders and hyperactivity. Adaptive and maladaptive behaviour was not related to the presence or absence of focal cortical PET abnormalities. However, adaptive behaviour scores showed an inverse correlation with the degree of metabolic abnormality in the frontal lobes.