This study focused on intersubjective relatedness with clients with Borderline Personality Disorder (BPD) within the field of Music Therapy. Clinical improvisation is increasingly recognized as a mean to develop or re-live patterns of mutuality, and by mutuality we mean the emergence of a polyphonic flow founded on the forces that constantly makes us balance between dependency and independency. Recent research addresses how people with BPD relate to others in Music Therapy. What is currently lacking is a detailed tracking of mutual aspects of the musical modality emerging in music therapy improvisations. This paper details research in this area, asking the question: in what ways do clients with BPD demonstrate capacities and difficulties to mutually attune to a music therapist in one piano improvisation?A Structured Piano Duet Improvisation paradigm (ABA’) was used that allowed periods of predictable interaction (A and A’), and more mutually attuned interaction (B). 20 BPD clients participated in one improvisation, at the beginning of their admission into hospital. The constant comparative method was used in which cycles of data collection, observation, coding and validation made it possible to abstract patterns of mutuality.Six categories of distorted patterns of mutuality were found: obsessive-rigid, associative-external, sensitive, agonistic, submissive, and aleatoric. Two dimensions were found to describe patterns of mutuality in improvisation: internal/external and affective/cognitive.The study identified a continuum of dysfunctions in intersubjective relatedness which can become a frame of reference to understand the interpersonal dynamics of a client in Music Therapy. This will help music therapists to be better prepared musically when working with BPD clients.