The aim of this study was to investigate etiological and nosological aspects of the schizotypal and borderline personality disorders. The sample consisted of 44 schizotypal, 15 schizotypal and borderline, and ten borderline same-sexed twin probands. The investigation of the co-twins indicated that genetic factors seemed to influence the development of the schizotypal, but not the borderline, personality disorders. The basic genetic core of the schizotypal syndrome seemed to consist of schizoid and paranoidlike features, and not psychoticlike cognitive and perceptual distortions. The study did not indicate any relationship between schizotypal and borderline personality disorders and affective and schizophrenic disorders. Further research is needed to confirm the independent status of the schizotypal syndrome in relation to the schizoid, avoidant, and paranoid personality disorders, and the borderline syndrome in relation to the histrionic , narcissistic, and antisocial personality disorders.