The Strengths and Difficulties Questionnaire (SDQ) is a brief behavioural screening questionnaire that asks about children's and teenagers' symptoms and positive attributes; the extended version also includes an impact supplement that asks if the respondent thinks the young person has a problem, and if so, enquires further about chronicity, distress, social impairment, and burden for others. Closely similar versions are completed by parents, teachers, and young people aged 11 or more. The validation study involved two groups of 5-15-year-olds: a community sample (N = 467) and a psychiatric clinic sample (N = 232). The two groups had markedly different distributions on the measures of perceived difficulties, impact (distress plus social impairment), and burden. Impact scores were better than symptom scores at discriminating between the community and clinic samples; discrimination based on the single "Is there a problem?" item was almost as good. The SDQ burden rating correlated well (r = .74) with a standardised interview rating of burden. For clinicians and researchers with an interest in psychiatric caseness and the determinants of service use, the impact supplement of the extended SDQ appears to provide useful additional information without taking up much more of respondents' time.