Objective: The present study explored the role of the therapeutic relationship and introject during the course of dialectical behavior therapy (DBT; Linehan, 1993) for the treatment of borderline personality disorder. Method: Women meeting "DSM-IV" criteria for borderline personality disorder (N = 101) were randomized to receive DBT or community treatment by experts. The Structural Analysis of Social Behavior (Benjamin, 1974) was used to measure both the therapeutic relationship and introject. Results: Relative to community treatment by experts, DBT participants reported the development of a more positive introject, including significantly greater self-affirmation, self-love, self-protection, and less self-attack, during the course of treatment and 1-year follow-up. The therapeutic relationship did not have an independent effect on intrapsychic or symptomatic outcome but did interact with treatment. DBT participants who perceived their therapist as affirming and protecting reported less frequent occurrences of nonsuicidal self-injury. Conclusions: The study showed positive intrapsychic change during DBT and emphasized the importance of affirmation and control in the therapeutic relationship. Results are discussed in the context of understanding the mechanisms of change in DBT. (Contains 1 footnote and 5 figures.)