Background Research has shown that children's beliefs about the causes of psychological problems are related to their attitudes and reactions towards affected peers. This study describes the development of the Children's Attributions about Psychological Problems in their Peers (CAPPP) Scale, which assesses children's beliefs about the causes of an internalizing and an externalizing condition.Methods The 16 items comprising the CAPPP are derived from previous qualitative research findings. Five hundred and ninety-five young people, drawn from five different age groups spanning early childhood to late adolescence, completed a CAPPP Scale for each of two vignettes describing the behaviour of hypothetical peers with attention deficit hyperactivity disorder (ADHD) and depression.Results Modifications following consideration of psychometric properties and conceptual fit resulted in a 12-item scale. For both the ADHD and depression conditions, the components that emerged were 'Volition','Recent Life Stress', 'Family Factors' and 'School Factors'.Conclusions The present study represents the first field trial of the CAPPP. Results suggest that children's and adolescents' beliefs about the causes of psychological problems are multidimensional and incorporate both individual and environmental factors.