Despite the well-established association between adolescent sexual activity and delinquent behavior, little research has examined the potential importance of relationship contexts in moderating this association. The current study used longitudinal, behavioral genetic data on 519 same-sex twin pairs (48.6 percent female) divided into two age cohorts (13-15 and 16-18 year olds) drawn from the National Longitudinal Study of Adolescent Health. Analyses tested whether adolescent sexual activity that occurred in romantic versus non-romantic relationships was associated with delinquency from adolescence to early adulthood, after controlling for genetic influences. Results indicated that, for both younger and older adolescents, common underlying genes influence both sexual behavior and delinquency. After controlling for these genetic influences, there was no within-twin pair association between sexual activity and delinquency in younger adolescents. In older adolescents, sexual activity that occurred in romantic relationships predicted lower levels of delinquency, both cross-sectionally and longitudinally, whereas sexual activity in non-romantic relationships predicted higher levels of delinquency. These results are consistent with emerging research that suggests that the psychological correlates of adolescent sexual activity may be moderated by the social context in which this activity occurs. (Published Abstract)