Background: In the context of much literature and conjecture about the relationship of personality disorders (PD) and major depressive disorder (MDD), this paper uses longitudinal data to assess the frequency with which PD patients, and especially those with borderline personality disorder (BPD), have recurrences (for patients with lifetime histories), or new onsets (for patients without lifetime histories) of MDD. Methods: A sample of 478 PD patients received reliable repeated follow-up assessments over a period of 6 years. The rates of new onsets and recurrences of MDD in all PD patients, and in BPD patients compared to OPD patients were analyzed. Whether age, gender, GAF score, or the number and types of BPD criteria predict new onsets or recurrences of MDD was also examined. Results: Eighty-five percent of PD subjects had episodes of MDD during the 6 year follow-up; of those with lifetime MDD, 85% had recurrences. Of the PD subjects without lifetime MDD, 44% had new onsets. BPD subjects were significantly more likely (p = .0036) to have recurrences of MDD but were about equally likely to have new onsets compared to OPD subjects. The number and types of BPD criteria were predictive of onsets and recurrences for all PDs, but were not more predictive for the BPD than OPD subsamples. Limitations: Longer term follow ups with a more epidemiologically representative sample of PDs would strengthen the generalizability of this study's findings. Conclusions: Having a comorbid PD confers significant risk for recurrences and for new onsets of MDD and confers a significantly negative effect on the course of MDD. BPD conferred more risk for recurrence than OPD.