There is emerging evidence of gene-environment interaction effects on conduct problems, both from adoption studies and from a study using a measured genotype. An association between non-violent family dysfunction and conduct problems has also been reported, although not in the context of gene-environment interaction studies. The aim of this study was to examine the interaction of genes and family dysfunction in contributing to conduct problems in young people. Parents of 278 monozygotic and 378 dizygotic twin pairs, aged 5-18, from the CaStANET birth cohort twin register were questioned about zygosity, conduct problems and family environment. Using structural equation modeling we tested for main and interactive effects of genes and family dysfunction modelled as an environmental "moderator variable". Both main and gene-environment interaction effects were highly significant. It was concluded that a risk genotype conferring susceptibility to family dysfunction is responsible for most of the variance in antisocial symptoms in childhood and adolescence.