British local authorities are required to monitor the mental health of looked after children using mean Strengths and Difficulties Questionnaire (SDQ) scores from parents or carers. This assumes that differences in mean SDQ scores reflect genuine differences in child mental health in this group, something we examined using nationally representative surveys (n = 1391, age 5-16). We found that the SDQ was a genuinely dimensional measure of mental health in these children and provided accurate estimates of disorder prevalence.