BACKGROUND For a long time there was controversy and reservations with regards to diagnosing borderline personality disorder (BPD) under the age of eighteen. Progress in scientific knowledge has shown that adolescence is a key phase in the understanding of the development of the disorder.<br/> AIM: To contribute to the understanding of the developmental trajectory of BPD by studying associations between BPD and the interpersonal functioning in adolescence.<br/> METHOD: To study associations between personality, cluster B personality pathology and BPD and associations between BPD and interpersonal functioning, specified in social information processing and relationship quality with parents and peers.<br/> RESULTS: Personality dimensions and social information processing, both independently and together, are related to cluster B personality pathology and BPD. Furthermore, the importance of quality of relationships with parents in relation to BPD is confirmed..<br/> CONCLUSION: The role of social environment is complex in the development of BPD. The results underpin the interpersonal nature of BPD in adolescence as a crucial phase within the psychosocial development.