Abnormal patterns of interpersonal behavior may represent a putative prodromal feature of major affective disorder (Hammen 2002). However, no studies have examined the naturalistic patterns of interpersonal behaviour among the adolescent offspring of parents with BD. The present study examined patterns of interpersonal functioning among 25 adolescents having a parent with BD (high risk) and 23 controls (low risk), between 15 to 25 years of age. Using event-contingent recording procedures in the natural environment, we assessed five dimensions of interpersonal behaviour: quarrelsomeness, agreeableness, dominance, submissiveness, and impulsivity. Behaviours were measured during social interactions occurring over 14 days. Multilevel modeling analyses indicated that the high risk group displayed more impulsive behaviours during social interactions than the low risk group, although this fell short of statistical significance ( t = -1.88, p < .056). Group by gender interactions revealed that high risk males displayed significantly more quarrelsome behaviours during social interactions than high risk females and control participants ( F = 7.49, p < .05). These results are consistent with previous findings suggesting that externalizing behaviour problems precede the development of major affective disorder, and indicate that impulsiveness and quarrelsomeness during social interactions might be an important developmental antecedent of mood disorders among the adolescent offspring of parents with BD.