Historically unresolved diagnostic questions regarding the borderline diagnosis and its relationship to schizophrenia are explored. Is the borderline syndrome a subvariant of schizophrenia or does it constitute a distinct diagnostic entity in its own right with specific, common, and identifiable features which set this group of patients off from others? Based on recent developments in psychoanalytic object relations theory, a Rorschach scoring manual designed to assess the primitive defenses that are conceptualized to underlie as well as organize the borderline patient was applied to independently selected samples of hospitalized borderline (DSM-III criteria) and schizophrenic (Research Diagnostic Criteria) adolescent and young patients. Consistent and significant differences were found between groups on the basis of defensive functioning. Borderline patients were found to use test measures of splitting, primitive devaluation, idealization, denial, and projective identification significantly more than schizophrenic patients. The levels of inter-rater reliability and the direction of the results in discriminating borderline from schizophrenic patients on the basis of defensive functioning indicate that the Rorschach Scoring System is a reliable and valid instrument. The implications of the results in terms of the differential diagnostic issues, the nature of borderline defenses, and the utilization of the Rorschach as a research instrument are considered.