The present study aims at determining the role for outcome of potential processes of change in psychotherapy for narcissistic personality disorder (NPD). They were examined on three levels: the content, the process, and the relationship. A total of 161 patients suffering with NPD were recruited in a naturalistic setting as part of the present study. They underwent a long- term clarification-oriented psychotherapy. Sessions 15, 20, and 25 were video- or audio-recorded and analyzed with an observer-rated instrument that measures the quality of the interaction processes from the patient's and therapist's perspectives. Different self-report measures were used to assess therapy outcomes. In-session improvement was observed in both patient and therapist processes across sessions. Patient improvement in the three levels of processes was systematically related with outcome. Only partial relationships were found between therapist improvement and outcome. The present study represents the first systematic insight into core changes in patients with NPD undergoing psychotherapy.