Borderline personality disorder (PD) has historically been cast as an unabating condition. Longitudinal data, however, support a more variable time course marked by remission and relapse. In the present study, we tested the possibility that borderline PD has both stable (i.e., consistently present across time and situation, as modern diagnostic systems stipulate) and dynamic (i.e., episodic and situational) elements. Participants were 668 patients from the Collaborative Longitudinal Personality Disorders Study who were administered semistructured diagnostic interviews 5 times over a decade. Trait–state-occasion modeling dissected borderline pathology into time-invariant (i.e., trait) and time-varying (i.e., state) components. Contradicting traditional views of PD intransigence, less than half of borderline PD variability (approximately 45%) was time-invariant (i.e., perfectly stable) over the study timeframe. Furthermore, we found that the time-invariant component of borderline pathology, which we termed borderline proneness, was very closely related (r = .81) to a previously validated Five Factor Model trait composite of borderline features. Moreover, the trait versus state components showed a clear pattern of discriminant validity in relation to several putative causal agents for borderline PD (i.e., environmental pathogens, temperament dimensions). We conclude that borderline pathology contains a stable core and sizable situational components, and that both elements relate systematically to normative personality dimensions and known risk factors. These findings have key implications for etiological research, prognosis, and treatment for borderline PD.