According to attachment theory, attachment style derives from social experiences throughout the life span. The authors tested this expectation by examining associations between the quality of observed interaction patterns in the family of origin during adolescence and self-reported romantic attachment style and observed romantic relationship behaviors in adulthood (ages 25 and 27). Family and romantic relationship interactions were rated by trained observers from video recordings of structured conversation tasks. Attachment style was assessed with items from D. W. Griffin and K. Bartholomew's (1994a) Relationship Scales Questionnaire. Observational ratings of warmth and sensitivity in family interactions were positively related to similar behaviors by romantic partners and to attachment security. In addition, romantic interactions characterized by high warmth and low hostility at age 25 predicted greater attachment security at 27, after controlling for attachment security at age 25. However, attachment security at age 25 did not predict later romantic relationship interactions after controlling for earlier interactions. These findings underscore the importance of close relationships in the development of romantic attachment security but do not indicate that attachment security predicts the quality of interactions in romantic relationships.