We investigated gesture dynamics by examining wrist-worn accelerometer data from 28 patient-therapist dyads involved in multiple sessions of mentalization-based therapy. We sought to determine if there were long-term correlations in the signals and evaluate the degree of complexity matching between patient and therapist. Moreover, we looked into the relationship between complexity matching and the level of therapeutic success (operationalized by change in mentalization and the severity of symptoms). The results indicated that the patient and therapist gesture dynamics were significantly different than long-term correlations produced by white noise. Further, six patient-therapist dyads matched each other in complexity across sessions, but no systematic relationship between the patient and therapists' was observed and there were no relationships between these dynamics and measures of therapeutic success.