The purpose of this study was to assess the role of biparental abuse and neglect in the development of borderline personality disorder (BPD). A semistructured research interview was used to blindly assess the childhood experiences of biparental abuse and neglect reported by 358 borderline inpatients and 109 axis II controls. Eighty-four percent of borderline patients reported having experienced some type of biparental abuse or neglect before the age of 18; 55% reported a childhood history of biparental abuse; 77% reported a childhood history of biparental neglect. These experiences were also reported by a substantial percentage of Axis II controls (biparental abuse or neglect [61%], biparental abuse [31%], and biparental neglect [55%]). However, borderline patients were significantly more likely than axis II controls to report having been verbally, emotionally, and physically but not sexually abused by caretakers of both sexes. They were also significantly more likely than controls to report having caretakers of both sexes deny the validity of their thoughts and feelings, fail to provide them with needed protection, neglect their physical care, withdraw from them emotionally, and treat them inconsistently. It was also found that female borderlines who reported a previous history of neglect by a female caretaker and abuse by a male caretaker were at significantly higher risk for having been sexually abused by a noncaretaker. Taken together, the results of this study suggest that biparental failure may be a significant factor in the etiology of BPD. They also suggest that biparental failure may significantly increase a preborderline girl's risk of being sexually abused by someone other than her parents.