There is no agreement in American psychiatry as to whether borderline should be regarded as a distinct entity. A review of the major view-points in the field reveals that borderline is variously used to designate: (1) a clinical disorder distinguishable by behavioral criteria; (2) a milder form of schizophrenia; (3) a nonspecific term encompassing several atypical affective disorders; (4) a psychostructural distinction. When the St. Louis approach to diagnostic validity is used as a guideline, the conclusion reached is that available data do not weigh conclusively for or against borderline's status as an independent entity. In particular, borderline, as defined by several investigators, appears distinct from schizophrenia, but requires further delimitation from the affective disorders.