Background.Although depression in childhood appears to be familial, transmission of symptoms could be genetic or environmental. Twin studies enable us to separate these effects. Our aim was to assess the importance of genetic and environmental factors on depressive symptoms during childhood and adolescence in an epidemiological sample of twins.Method.Questionnaires (including the Mood and Feelings questionnaire) were mailed to a systematically ascertained sample (411 twin pairs) aged between 8 and 16 years. The overall response rate was 77%.Results.Data for the whole sample were best explained by an additive genetic model, with heritability of depressive symptom scores estimated at 79%. However, on splitting the sample, symptoms in children (aged 8 to 11) could be explained by shared environmental factors only, while symptoms in adolescents (aged 11 to 16) remained highly heritable.Conclusions.The results suggest that although depressive symptoms appear to be largely heritable, the influence of genetic and environmental factors may vary with age.