This study evaluated the relationship between traumatic brain injury (TBI) and borderline personality disorder (BPD). Forty-three males with BPD were compared with 49 age- and sex-matched controls with other psychiatric diagnoses. The occurrence of TBI was identified by chart review. Head injury was identified in 42% of patients meeting criteria for BPD but in only 4% of controls (p < 001). All patients but one had sustained a TBI prior to the diagnosis of BPD. The prevalence of TBI is higher in subjects with BPD than in controls. Since TBI occurred prior to the full expression of the BPD clinical syndrome, TBI appears to be a cause rather than the result of BPD.