There is considerable interest in relations between sexual behavior and romantic attachment styles in adolescence as attachment needs are increasingly met through intimate partners rather than parents. The objectives of this research were to organize a measure of sexual behavior within an attachment theory framework, and then show that this new measure uniquely predicted sexual approach styles and invasive sexual experiences. 190 18- and 19-year-old university students in late adolescence completed sexual behavior items that were provided ambivalent (anxious) and avoidant dimensions. These were systematically related to the romantic attachment dimensions of the "Experiences in Close Relationships--Revised". However, even after romantic relationship style, gender, and social desirability were controlled, avoidance in sexual relationships predicted lifetime sexual partner number and negatively predicted positive sexual strategies, and ambivalence in sexual relationships predicted invasive and coercive sexual behaviors. A measure specific to sexual relationships informs the attachment and romantic context of sex in late adolescence. (Contains 4 tables.)