Self-reported anxiety and depression symptoms in children and adolescents have been shown to be heritable, and are also highly correlated. Furthermore, there have been indications in the literature of sex and age differences in the aetiologies of these two types of symptoms. This study set out to ascertain to what extent the genetic and environmental factors that influence anxiety symptoms also influence depression symptoms, and whether these are the same in children and adolescents, and males and females. Four hundred and ninety pairs of twins aged 8 to 16 years completed the Children's Depression Inventory and the Trait scale of the State-Trait Anxiety Inventory for Children. There were significant effects of age and sex on the variance in and covariance between these two types of symptom. Bivariate genetic analyses of the measures indicated that the genetic influences on anxiety and depression were shared for all four groups, a finding that has been consistently demonstrated for adults.