Previous research has identified an invalidating childhood environment and a biological predisposition for affective instability to be two precursors for the development of borderline personality disorder (BPD). In addition, rumination, particularly in response to anger, is significantly associated with symptom severity and dysregulated behavior in BPD. This study examined whether a significant relationship between childhood precursors and current BPD symptom severity could be accounted for by the tendency to engage in anger rumination in a sample of undergraduate students (N = 342), including many with high BPD features. Results showed a significant indirect effect of anger rumination in the relationship between self-reported childhood emotional vulnerability (but not invalidating childhood environment) and BPD symptom severity. This finding suggests that anger rumination could be a risk factor for BPD. These results suggest the importance of continued research on the role of anger rumination in the development and exacerbation of BPD symptoms.