Abstract.It is important to investigate the outcomes of psychotherapy and, especially, its process. Regarding child psychodynamic psychotherapy, available studies are in smaller numbers. Therefore, we still do not fully know the mechanisms of change in treatments with this age group. The Child Psychotherapy Q-Set (CPQ) was designed to analyze the therapeutic process with children. It permits one to identify interaction structures (i.e., repetitive patterns of interaction) and how they change in the course of a treatment. Based on these assumptions, the aim of this study was to identify and to analyze the interaction structures in the psychodynamic therapy of a boy diagnosed with Asperger's disorder and possible changes in his psychic organization. A mixed longitudinal study, based on the Systematic Case Study procedure, was performed. Approximately 30 months of the boy's psychotherapy were analyzed using the CPQ. The Rorschach method was used as the outcome measure. Four interaction structures were identified, using the CPQ: Active, confident and lively child, competing with connected, mentalizing and accepting therapist; Withdrawn and defensive child with uncertain, unresponsive and didactic therapist; Accepting therapist with demanding, provocative and hostile child; and Reassuring, supportive, nondirective therapist with a compliant and not spontaneous child. Two interaction structures varied over time. Some changes in Rorschach variables were detected after two years of treatment.