As part of a larger study of pathological self-injury, 14 self-mutilators and 14 psychiatric controls matched for age, sex, and inpatient/outpatient status were administered the Diagnostic Interview for Borderlines. Consistent with Gunderson's theory of borderline personality disorder, the self-mutilators scored significantly higher on impulse-action patterns, affects, psychoticism, and interpersonal relations as well as on the total borderline index. A review of clinical records likewise revealed that self-mutilators were more likely than controls to have been diagnosed as borderline and to have received a greater number of different diagnoses during their treatment career. The results have implications for developmental theory, diagnosis, and treatment and provide support for the construct validity and clinical utility of the borderline syndrome and the Diagnostic Interview for Borderlines.