Borderline personality disorder (BPD) is a common psychiatric disorder that is often overlooked in treatment settings. This report describes the validation of a new self-report screening measure for DSM-IV BPD--the McLean Screening Instrument for Borderline Personality Disorder (MSI-BPD). Two hundred subjects with treatment histories whose ages ranged from 18 to 59 filled out the MSI-BPD. Each subject was then interviewed, blind to MSI-BPD results, with the BPD module of the Diagnostic Interview for DSM-IV Personality Disorders (DIPD-IV; Zanarini, Frankenburg, Sickel, & Yong, 1996). Of these 200 subjects, 139 (69.5%) met DSM-IV criteria for BPD as assessed by the BPD module of the DIPD-IV and the remaining 61 subjects (30.5%) did not. Using logistic regression analyses, an MSI-BPD cutoff of 7 or more of the measure's 10 items was judged to be the best cutoff. This was so because it yielded both good sensitivity (.81; percentage of correctly identified cases) and specificity (.85; percentage of correctly identified noncases) for the diagnosis of DSM-IV BPD. For younger subjects, diagnostic efficiency was even greater. For example, sensitivity was .90 and specificity was .93 at a cutoff of 7 for the 63 subjects who were 25 years old or younger. The results of this study suggest that the MSI-BPD may be a useful screening instrument for the presence of DSM-IV borderline personality disorder.