The 1999 British Child and Adolescent Mental Health Survey, a nationwide epidemiological study of rates of psychiatric disorder in children aged 5 to 15 years, provided the opportunity to investigate the mental health of children with epilepsy. These children and their families experience disability specifically because of additional emotional, behavioural, and relationship problems, and this is the first epidemiological study that directly measures these impairments. Information was obtained by interviewing a main carer and teacher for 10316 children; 67 children with epilepsy were identified (35 males, 32 females; mean age 10 years 2 months, SD 2 years 11 months, range 5 to 15 years), and compared with the 47 children with diabetes (27 females, 20 males; mean age 10 years 4 months, SD 3 years 4 months, range 5 to 15 years) and 10202 controls (50% male; mean age 9 years 11 months, SD 3 years 1 month, range 5 to 15 years). DSM-IV psychiatric diagnoses were derived from the Development and Well-Being Assessment in combination with the interview and a specialist clinician rating. Parental reports of emotional and behavioural problems, their impact, and associated peer problems were also obtained. Rates of psychiatric disorder were 37% (95% confidence interval [CI] 22 to 49) in epilepsy, 11% (95% CI 2 to 19%) in diabetes, and 9% (95% CI 9 to 10%) in control children. Parents of children with epilepsy consistently reported more problems, with greater impact and associated peer problems. Epilepsy, but not diabetes, was independently (adjusted for age, sex, and severe learning difficulties) associated with all behavioural variables in regression analyses. Emotional, behavioural, and relationship difficulties are common in children with epilepsy, and constitute a significant burden to the children and their families, indicating the need for effective mental health services for these children.