OBJECTIVE: Numerous studies based on parent questionnaires reveal that obese children are subject to both physical and psychosocial strains. Children are not included as informants in the assessment procedure, and there is a lack of clinical interview studies assessing DSM-IV-based diagnoses in overweight children. METHODS: Emotional and behavioural problems of treatment seeking obese children (N = 59; aged 8-12-years) attending a mental health service were assessed by means of clinical interviews (Kinder-DIPS) that included child and parent reports. In addition, Child Behavior Checklist (CBCL) data (N = 55) available for this clinical sample were compared with data of a representative sample of normal-weight children (N = 1,080). Furthermore, CBCL data of the clinical subsample of 10-12 year-old overweight children (N = 34) were compared with data of a subsample of 10-12 year-old overweight children (N = 33) and of the subsample of normal-weight (N = 386) children from a representative population, both matched by sex. RESULTS: 23 children (39%) met full criteria for a psychiatric disorder according to DSM-IV, and 19 (34.5%), nearly one third, suffered from Binge Eating Disorder (BED). The overweight clinical sample and their normal weight peers differed significantly on 6 of the 8 primary syndrome scales, on the internalizing and the externalizing scores, and on the total problem score of the CBCL. The overweight children in the clinical sample scored significantly higher than the overweight children in the representative sample on the subscale measuring social problems and the total score, with BMI accounting for the difference in total score. Within the representative sample, overweight 10-12 year-old children scored significantly higher than their normal weight peers on two scales measuring delinquency and social problems. CONCLUSION: A third of the obese children had mental disorders and behaviour problems. This finding applies to clinically referred obese children and, to a much lesser degree, to nonclinical obese subgroups in the normal population. Clinically obese children constitute a patient group that is in urgent need of multimodal treatment. As a consequence, a major change in treatment and prevention of childhood obesity is warranted, i.e., from the mere treatment of eating and activity behaviours to the inclusion of treatment of co-morbid mental disorder and prevention of chronification.