The purpose of this study was to determine the percentage of borderline patients who first engaged in self-mutilation as children and to compare the parameters of their self-harm to those of borderline patients who first harmed themselves at an older age. Two hundred and ninety inpatients meeting both Revised Diagnostic Interview for Borderlines (DIB-R; Zanarini, Gunderson, Frankenburg, & Chauncey, 1989) and Diagnostic and Statistical Manual of Mental Disorders (3rd ed. ref.) (DSM-III-R; APA, 1987) criteria for borderline personality disorder were interviewed about their history of self-mutilation. Of the 91% with a history of self mutilation, 32.8% reported first harming themselves as children (12 years of age or younger), 30.2% as adolescents (13-17 years of age), and 37% as adults (18 or older). Using logistic regression analyses and controlling for baseline age, it was found that those with a childhood onset reported more episodes of self-harm, a longer duration of self-harm, and a greater number of methods of self-harm than either those with an adolescent or adult onset to their self-mutilation. The results of this study suggest that a sizable minority of borderline patients first engage in self-harm as children and that the course of their self-mutilation may be particularly malignant.