Background: Psychotherapy is an important part of the addiction treatment, especially the group therapy with related changes, through which a patient overcomes during the treatment. Goal: The aim of the quantitative research was to test if a change has occurred in particular areas measured by the questionnaire CORE-OM. Methods: A research hypothesis was set that assumed statistically significant differences in particular dimensions of the questionnaire measuring the psychotherapeutic change at the beginning and at the end of the addiction treatment. The sample consisted of 56 participants (M = 43,52; SD = 12,09). The CORE-OM questionnaire was administrated. The questionnaire consists of 34 items and has three main dimensions – subjective well-being, problems/symptoms, interpersonal and social functioning. Remaining items form the addition dimension - risk behavior. Results: We verified hypothesis about the existence of difference in changes between the beginning and the end of the treatment by parametric and nonparametric tests. The outcomes of the analysis proved that the respondents have improved through the addiction treatment in their subjective well-being and interpersonal/social functioning, the degree of problems/symptoms has decreased. The difference in risky behavior was not proved. Conclusion: Outcomes correspond with the practical knowledge and also with research findings of the several authors. The research in therapeutic change of the patients in treatment brought valuable knowledge for verification of the therapeutic process effectiveness in Drug Addiction Treatment Centre Banská Bystrica.

Key words: addiction treatment – psychotherapy – therapeutic change