This report contains data from the first national survey of the mental health of young people aged 5-17 years, who are being looked after by local authorities in Wales. The survey was commissioned by the Welsh Assembly Government and fieldwork took place between October 2002 and June 2003. The primary aim of the report is to identify the prevalence of mental health problems in terms of three main categories: conduct disorder, hyperactivity, and emotional disorders by child and placement characteristics. It also looks at the impact of children’s mental health problems in terms of social impairment and adverse consequences for others; as well examining service use. Topics discussed include: survey methodology; concepts in assessing childhood mental disorders; prevalence of mental disorders; physical health and service use; education, social participation and lifestyle behaviours including smoking, drinking and drug use. Of the 5-17 year olds looked after by local authorities in Wales, 49 per cent were assessed as having a mental disorder (similar to prevalence rates in England and Scotland).