The present research assessed attachment style and gender differences in adolescents' relationships with the opposite sex, using questionnaire and diary methodologies. In Study 1, 193 unmarried undergraduate subjects completed questionnaire measures of attachment style, relationship history and attitudes to sex. Relationship history data supported previous findings: avoidant subjects reported fewer and less intense love experiences; anxious/ambivalent subjects reported frequent but less enduring love relationships; secure subjects reported more loving and satisfying relationships. As hypothesized, avoidant subjects were more accepting of casual (uncommitted) sex than other attachment groups. In Study 2, 85 subjects from the previous study used a diary format to record their interactions with members of the opposite sex, classifying each interaction according to intimacy level and partner type. Avoidant subjects engaged in fewer interactions overall, fewer chats, and interacted with a smaller number of friends. Anxious/ambivalent subjects engaged in fewer interactions with strangers than did secure subjects. Female avoidants and male anxious/ambivalents were the least likely to report engaging in sexual intercourse during the course of the study, suggesting that attachment style and gender role expectations jointly influence relationship development.