Background: The consideration of impairment plays a crucial role in detecting significant mental health problems in children whose symptoms do not meet diagnostic criteria. The assessment of impairment may be particularly relevant when only short screening instruments are applied in epidemiological surveys. Furthermore, differences between childrens' and parents' perceptions of present impairment and impairing symptoms are of interest with respect to treatment-seeking behaviour.

Objectives: The objectives were to assess parent- and self-reported impairment due to mental health problems in a representative sample of children and adolescents; to describe the characteristics of highly impaired children with normal symptom scores; and to investigate the associations between symptoms in different problem areas and impairment.

Methods: The mental health module of the German Health Interview and Examination Survey for Children and Adolescents (the BELLA study) examined mental health in a representative sub-sample of 2,863 families with children aged 7-17. Self-reported and parent-reported symptoms of mental health problems and associated impairment were identified by the extended version of the strengths and difficulties questionnaire (SDQ) in children 11 years and older.

Results: Considerable levels of distress and functional impairment were found with 14.1% of the boys and 9.9% of the girls being severely impaired according to the parental reports. However, self-reported data shows a reversed gender-difference as well as lower levels of severe impairment (6.1% in boys; 10.0% in girls). Six percent of the sampled children suffer from pronounced impairment due to mental health problems but were not detected by screening for overall symptoms. Childrens' and parents' reports differed in regard to the association between reported symptom scores and associated impairment with children reporting higher impairment due to emotional problems.

Conclusions: The assessment of impairment caused by mental health problems provides important information beyond the knowledge of symptoms and helps to identify an otherwise undetected high risk group. In the assessment of impairment, gender-specific issues have to be taken into account. Regarding the systematic differences between childrens' and parents' reports in the assessment of impairment, the child's perspective should be given special attention.