Although childhood anxiety appears to aggregate in families, transmission could be explained by both genetic and shared environmental factors. Twin studies can be used to disentangle genetic and environmental effects. In this study, a systematically ascertained sample of twins was used to investigate whether anxiety symptoms are heritable. Parent-rated anxiety symptoms could best be explained by an additive genetic model with heritability estimated at 59%. However, when self ratings were analysed (in the adolescent subsample), familial transmission could be accounted for by shared environmental factors only.