Although stability and pervasive inflexibility are general criteria for Diagnostic and Statistical Manual of Mental Disorders, 4th edition (American Psychiatric Association, 1994) personality disorders (PDs), borderline PD (BPD) is characterized by instability in several domains, including interpersonal behavior, affect, and identity. The authors hypothesized that such inconsistencies notable in BPD may relate to instability at the level of the basic personality traits that are associated with this disorder. Five types of personality trait stability across 4 assessments over 6 years were compared for BPD patients (N = 130 at first interval) and patients with other PDs (N = 302). Structural stability did not differ across groups. Differential stability tended to be lower for 5-factor model (FFM) traits in the BPD group, with the strongest and most consistent effects observed for Neuroticism and Conscientiousness. Growth curve models suggested that these 2 traits also showed greater mean-level change, with Neuroticism declining faster and Conscientiousness increasing faster, in the BPD group. The BPD group was further characterized by greater individual-level instability for Neuroticism and Conscientiousness in these models. Finally, the BPD group was less stable in terms of the ipsative configuration of FFM facet-level profiles than was the other PD group over time. Results point to the importance of personality trait instability in characterizing BPD.