The authors extended previous work on the hypothesis that borderline personality disorder (BPD) can be understood as a maladaptive variant of personality traits included within the 5-factor model (FFM) of personality. In each of 3 samples, an empirically derived prototypic FFM borderline profile was correlated with individuals' FFM profiles to yield a similarity score, an FFM borderline index. Results across all samples indicated that the FFM borderline index correlated as highly with existing borderline measures as they correlated with one another, and the FFM borderline index correlated as highly with measures of dysfunction, history of childhood abuse, and parental psychopathology as did traditional measures of BPD. Findings support the hypothesis that BPD is a maladaptive variant of FFM personality traits.