Past social network analysis studies have indicated that patients with borderline personality disorder (BPD) are significantly more socially isolated than comparison subjects. The present study aimed to build on the findings of these cross-sectional social network analysis studies. The first purpose of this study was to assess and compare the prevalence of social isolation in borderline patients and personality-disordered comparison subjects over 20 years of follow-up. The second was to determine the best baseline predictors of social isolation in these borderline patients. A total of 290 adult inpatients meeting rigorous criteria for BPD and 72 personality-disordered comparison subjects were recruited during inpatient admission at the participating institution. At baseline, interviews assessing psychiatric diagnoses, psychosocial functioning, and childhood history, and a self-report questionnaire assessing temperament were administered to all subjects. The diagnostic and psychosocial measures were readministered every 2 years over the course of 20 years. It was found that borderline patients were significantly more likely to be socially isolated than personality-disordered comparison subjects over time. Additionally, among borderline subjects, three variables were found to be significant multivariate predictors of social isolation: lower childhood competence, lower trait extraversion, and lower trait agreeableness. Taken together, these results suggest that social isolation remains an unfortunate outcome in a sizable minority of borderline patients over time. These results also indicate that isolation is strongly associated with enduring aspects of competence and temperament in patients with BPD. (PsycINFO Database Record (c) 2019 APA, all rights reserved).