Most twin and adoption studies of conduct problems have demonstrated modest genetic effects but substantial contributions of shared family environment. Conversely, most investigations have shown marked genetic influences but modest contributions of shared family environment in adult antisocial behavior. However, most previous work has focused on male subjects. We obtained retrospective reports of DSM-III-R-defined conduct disorder (CD) and adult antisocial behaviors from a population-based sample of female-female twin pairs. Genetic and environmental contributions to conduct problems and adult antisocial behaviors were examined using polychoric correlation coefficients and univariate structural equation modeling. Statistically significant but modest heritability was observed for conduct problems. A small, statistically nonsignificant contribution of shared family environment to CD behavior was also noted. Adult antisocial symptoms showed modest contributions of both additive genetic and shared family environmental factors. In both childhood and adulthood, the largest influence on antisocial behavior was individual-specific environment. Our findings support the importance of both genetic and environmental factors in antisocial behavior among women as well as the possibility that the relative importance of each set of influences differs by sex in both childhood and adulthood.