Objective: To determine if the substance use patterns of one's close friends and romantic partners would be a significant contributor to the co-occurrence of borderline personality disorder (BPD) features and drug use problems above and beyond impulsivity and negative emotionality. Participants: Participants were 2,202 undergraduates who attended a large southeastern university between 2003 and 2006. Methods: All subjects completed measures assessing the presence of BPD symptoms, drug use problems, general personality traits, and the proportion of their friends and partners who had used illicit drugs within the past 12 months. Results: The illicit drug use patterns of one's friends and romantic partners was a significant contributor to the co-occurrence of BPD features and drug use problems even when controlling for levels of impulsivity and negative emotionality. Conclusions: These findings suggest possible additional avenues for treatment focusing on helping students with BPD and drug use problems modify their social group. (Contains 1 figure and 2 tables.)