Background: Poor interpersonal functioning may represent a putative prodromal feature of major affective disorder. However, no studies have examined the naturalistic patterns of social behaviours among the offspring of parents with a major affective disorder. The present study assessed daily social interactions among 25 offspring of parents with bipolar disorder and 23 control participants in late adolescence and young adulthood., Methods: Using event-contingent recording procedures, interpersonal behaviours and perceptions were assessed along four scales (quarrelsomeness, agreeableness, dominance, and submissiveness) and were measured during specific social interactions over 14 days., Results: Multilevel modeling analyses revealed no group differences on any of the four scales, but gender by group interactions were was observed. High-risk males reported higher mean levels of quarrelsome behaviour and lower mean levels of agreeable behaviour than high-risk females, whereas low-risk males and females reported comparable levels of affiliative behaviours. High-risk participants reported more externalizing, but not internalizing, problems on the Achenbach Youth Self-Report Form than low-risk participants., Limitations: Although event-contingent recording reduces the self-report bias associated with self-report questionnaires, participants may have been biased in the selection of interactions they chose to record., Conclusions: Overall, the offspring of parents with BD, relative to controls, report no deficits in social functioning in the natural environment. However, high-risk youth displayed elevated externalizing problems and gender-specific patterns of social functioning that may precede the development of major affective disorder.