Background: High intelligence may be associated with positive (adaptive, desired) outcomes, but may also come with disadvantages. Aims: To contribute empirically to the debate concerning whether a trade-off in IQ scores exists in relation to attention-deficit hyperactivity disorder (ADHD) and related problems, suggesting that high intelligence - like low intelligence increases the risk of ADHD. Method: Curves of the relation between IQ score and ADHD problems were fitted to questionnaire data (parent, teacher, self-report) in a population-based study of 2221 children and adolescents aged 10-12 years. Externalising and internalising problems were included for comparison purposes. Results: Higher IQ score was most strongly related to fewer attention problems, with more rater discrepancy in the high v. average IQ range. Attention problems - but only minimally hyperactivity/impulsivity problems - predicted functional impairment at school, also in the higher IQ range. Conclusions: Attention problems in highly intelligent children are exceptional and affect school performance; they are therefore a reason for clinical concern.