State and trait anxiety define different aspects of anxiety, and may represent environmentally and genetically mediated components of this phenotype. Furthermore their relationship, where trait anxiety is expressed through levels of state anxiety under threatening circumstances, may represent a process of interplay between a genetic vulnerability factor and an environmental stressor. To test these hypotheses, we explored genetic and environmental influences on measures of state and trait anxiety in a sample of 1058 twins (521 males and 537 females) aged 8–16. The results were consistent with these hypotheses. State anxiety is largely influenced by environmental factors in males and females whereas trait anxiety shows moderate genetic effects and substantial non-shared environment effects. Their association was accounted for by non-shared environmental effects, with modest genetic and shared environmental inputs. The implications of these results for vulnerability mechanisms involving stress reactivity on anxiety are discussed.