This study investigated the relationship between affect attunement and subsequent improvement in attachment insecurity in adult psychotherapy. Particular attention was given to nonverbal aspects of therapist affect attunement, as defined and measured with the Affect Attunement Scales (AAS; Svartberg, 2005). Forty-nine patients diagnosed with cluster C personality disorders were randomly assigned to 40 sessions of short-term psychodynamic psychotherapy or cognitive therapy. Based on patients' self-reports on the Inventory of Interpersonal Problems (Horowitz, Rosenberg, Baer, Ureño, & Villaseñor, 1988), attachment styles and attachment security were determined, following procedures described by Hardy and Barkham (1994). On the basis of audio data from video recordings of an early session, trained raters used the AAS to determine level of therapist attunement. Results showed that initial higher levels of nonverbal matching of affect (one of the AAS scales) by the therapist predicted a decrease in avoidant attachment style at termination, whereas nonverbal matching of affect as well as nonverbal openness and regard for the patient's experiences (another AAS scale) predicted a decrease in ambivalent attachment style. In contrast, verbal aspects of attunement did not predict attachment outcome when the influence of nonverbal aspects were taken into account. Being consistent with findings in infant studies (Beebe et al., 2000), our findings regarding nonverbal attunement lend support to the intimate connection between nonverbal affect attunement and attachment security across the life span.