From the perspective of attachment theory (Bowlby, 1988), this study examined if client attachment to therapist developed over the course of psychotherapy and if changes in attachment to therapist were associated with treatment outcomes. Clients (N = 112), receiving psychodynamic therapy from trainee therapists (N = 29), completed the Client Attachment to Therapist Scale (Mallinckrodt, Gantt, & Coble, 1995) and the Outcome Questionnaire-45 (Lambert et al., 1996) at baseline and every eighth session. Multilevel linear growth curve analyses showed that secure attachment to therapist increased and avoidant-fearful attachment to therapist decreased. Multilevel linear regression showed that when within-client secure attachment to therapist was higher, subsequent symptoms improved more. Client-level and therapist-level effects were explored. Results suggest that the development of a secure attachment to therapist is important for positive treatment outcomes. Implications for practice and research are discussed.