A new self-rated scale to measure severity and change in persons with borderline personality disorder (BPD) is described. The Borderline Evaluation of Severity Over Time (BEST) was developed to rate the thoughts, emotions, and behaviors typical of BPD. Data were collected in the course of a randomized controlled trial (RCT) of Systems Training for Emotional Predictability and Problem Solving (STEPPS) for subjects with BPD. The instrument showed moderate test-retest reliability, high internal consistency, and high discriminant validity. Its 15 separate items showed a moderate or better correlation with the total score. The BEST was also sensitive to clinical change as early as week 4 of the RCT and correlated highly with other measures of illness severity. We conclude that the new scale is both reliable and valid in measuring severity and change in persons with BPD.