On the basis of a selected review of some important theoretical discussions and empirical investigations of patients characterized as borderline, a self-report questionnaire, the Borderline Syndrome Index (BSI), was constructed. Its purpose was to provide an assessment of the borderline syndrome that would increase the amount of information rapidly available to the clinician. The items reflect criteria cited in the literature as important characteristics of borderline patients and also incorporate the criteria for the borderline personality organization as listed in the DSM-III draft. The BSI was completed by 50 normal individuals, 36 nonpsychotic depressed outpatients, 35 patients diagnosed as borderline, and 20 inpatients diagnosed as schizophrenic. The internal consistency of the 52-item BSI was .92. It significantly discriminated the borderline patients from the normal individuals, from the depressed outpatients, and from the schizophrenic inpatients, thereby providing a measure of discriminative validity. An item analysis indicated that the most discriminating items were concerned with impaired object relations, impulsivity, emptiness and depression, depersonalization, and lack of self-identity. Cross-validation of the BSI was carried out on independent samples of borderline and nonborderline patients, and it was found to discriminate significantly between the two. Preliminary percentile norms are presented in order to provide clinicians with additional information to assist them in their diagnostic assessments.