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Background: Differentiation of borderline personality disorder (BPD) and bipolar I disorder (BD) has been challenging. The assessment of shared symptoms in the context of the overall personality functioning, the patient's sense of self, and the quality of his object (interpersonal) relations is proposed to be valuable for the differential diagnosis of these disorders., Methods: We empirically investigated the level of personality organization (PO), identity integration, and quality of object relations in patients suffering from BD or BPD using the Structured Interview of Personality Organization (STIPO) and the Level of Personality Functioning Scale (LPFS) in 34 BPD and 28 BD patients as well as 27 healthy control persons. Group comparisons and a logistic regression model were calculated to analyze group differences., Results: The BPD group showed significantly greater impairment in several domains of personality functioning, namely "identity", and "self- and other-directed aggression", while showing lower impairment in "moral values". The overall level of PO in the BPD group was significantly lower when excluding not only BPD but any personality disorder (PD) in the BD sample. Severity of impaired personality structure had a major impact on symptom load independent of the main diagnosis BD or BPD., Conclusions: Our data show greater impairment in personality functioning in BPD than in BD patients. BD patients present with varying levels of PO, whereas in BPD severe deficits in PO are pathognomonic. The level of PO has a significant impact on symptom severity in both BD and BPD patients. Therefore, careful assessment of PO should be considered for differential diagnosis and adequate treatment planning., Competing Interests: Declarations. Ethics approval and consent to participate: The study was reviewed and approved by the Ethics Committee of the Medical University of Vienna (Approval number: 1116/2015). Consent for publication: Written informed consent was obtained from all participants. Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)