THIS STUDY documents dramatic improvements in patients with borderline personality disorder (BPD) and explores their possible determinants. From a sample of the 160 carefully diagnosed borderline patients on whom prospective follow-along data was collected, a subsample of 18 was identified whose DSM-IV criteria count fell to two or fewer during the course of the first 6 months of the study and retained that improvement for the next 6 months. Follow-along data including month-by-month ratings of BPD criteria; week-by-week ratings of Axis I disorders, medication changes, and life events were then used to establish concensus ratings on four hypothesized causes: Axis I remissions, situational change, misdiagnosis, and treatment effects. Follow-up data collected at 2 years was examined to see whether the improvements persisted. The results were that 18 BPD patients underwent dramatic improvements in the first 6 months. Only one had relapsed by 2 years. Though one was judged to have been misdiagnosed at baseline, the most important determinants were judged to be situational changes (n = 10) and remissions of co-occurring Axis I disorders (n = 7). In 10 patients treatment appeared to have facilitated these situational or Axis I resolutions. In conclusion, patients with BPD can make significant improvements that are rapid and of sufficient duration to be considered remissions. Determinants were identified that warrant further prospective evaluation.