A single case study for a psychoanalytic psychotherapy process in a girl and her mother is presented from the perspective of the Object Relations Theory framework. Of interest are the empirical findings of therapeutic change and the effectiveness of the tool using a method created by a team of researchers from a university chair. A qualitative and quantitative analysis of textual observational records of sessions was conducted using codes constructed ad hoc from specific manuals for children and adults. The results of the application of the method are contrasted with results from psychological assessment techniques. Therapeutic change indicators are linked to progress in the processes of symbolization; therapeutic efficacy is recorded when the psychotherapist’s interventions favour symbolization and mentalization processes.