The paper illustrates the relational nature of the psychiatric category of borderline personality disorders based on the analysis of symptomatic criteria set out in the International Classification of Diseases and Related Health Problems, 11th Revision (ICD-11) issued by the World Health Organization (WHO), as well as the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) developed by the American Psychiatric Association (APA). We point to the appreciation of the relational nature of the phenomenon, which is noticeable already in a very cursory analysis, in the latest WHO and APA classification systems. The clinical manifestations of these criteria seem not only to be interconnected, but also to result from one another, suggesting a fundamentally structural nature of the phenomenon. All the described difficulties relate to interpersonal relationships – both in terms of the direct causes of disfunctions and long-term family and social consequences. This particularly applies to the borderlinetype emotionally unstable personality disorder, which is inseparably linked to emotional volatility and instability, and which is difficult to define using criteria in a way that does not give rise to diagnostic challenges. Therefore, we draw attention to difficulties with relationships, which are an important diagnostic criterion. Both ICD-11 and DSM-5 contain separate diagnostic categories for relationship problems, which, as it turns out, not necessarily coexist with other disorders. Linking relationship pathology with personality disorders may be of great importance for establishing the correct diagnosis and therefore choosing an appropriate treatment strategy.