There is some evidence that adolescent romantic involvement is associated with delinquent behaviour. One aim of this longitudinal study was to determine whether this holds for romantic relationships deemed important by the participants. A second aim was to test whether this association was stronger for adolescents with pre-existing delinquent behaviour and personality traits of impulsivity and thrill seeking (delinquency propensity). Sex differences also were examined. Participants were 686 7th and 8th grade students who completed three assessments over three years. The results showed that delinquency was associated with earlier romantic relationships among those who were higher in delinquency propensity one year earlier. This association was stronger among girls than boys. Thus, romantic relationships amplified girls' and boys' existing delinquency propensity, but this was strongest among girls. (Contains 1 table and 2 figures.)