We examined the reliability and validity of scores on an interpersonal measure of borderline personality disorder (BPD). Ratings on the Interpersonal Measure of Borderline Personality Disorder (IM-B) were based on nonverbal behaviors and interpersonal interactions occurring during clinical interviews with 276 adults. Scores on the IM-B exhibited good reliability. IM-B scores also displayed expected patterns of associations with scores on other measures of BPD, as well as with scores on measures of affective dysfunction, interpersonal pathology, and behavioral impairment associated with BPD, including indexes of maladaptive emotion regulation, interpersonal sensitivity, and self-harm. The pattern of associations for IM-B scores was quite similar to what would be expected for a dimensional measure of BPD symptoms. Scores on the IM-B were also associated with symptoms of disorders generally comorbid with BPD. Finally, IM-B scores contributed incrementally to the prediction of interpersonal dysfunction and suicidal ideation and behavior. Discussion focuses on implications for the assessment of BPD.