Introduction: Despite many studies on the treatment of pathological gamblers, there is no clearly defined and generally accepted psychotherapeutic approach and there is no standardized therapeutic program of proven efficacy. In Croatia, healthcare institutions today do not have enough services to deal exclusively with pathological gamblers. The treatment of pathological gambling that develops backwards for several years, is implemented through Gambling Addicts Club (KLOK) and represents the psychosocial treatment with the multimodal therapeutic approach. Aims of the research: The aims of this research are to (1) examine the characteristics of pathological gamblers in the treatment of the Gambling Addicts Club; (2) examine the changes in psychosocial functioning after six months of treatment; and (3) examine the pathological gamblers’ perception of the KLOK treatment. Patients and methods: The sample consists of N=64 pathological gamblers (Mage=35.72; SDage=10.59) that are included in the Gambling Addicts Club treatment in the City of Zagreb (KLOK). In order to conduct this research, a wide battery of instruments was used: (1) Questionnaire about general and socio-demographic data, (2) Questionnaire about history of disease and treatment, (3) The self-assessment questionnaire of gambling addiction according to DSM-5 criteria (4) Questionnaire of gambling activities, (5) Individual questions about gambling modalities and addiction development processes, (6) Gambling related cognitive distortion scale, (7) Questionnaire of perceived/necessary knowledge/skills required for success in the games of chance, (8) Check-list of gambling-caused financial problems, (9) Question of the current stage of change, (10) Check-list of the substance use frequency, (11) Check-list of mental health problems, (12) Behavior and Symptom Identification Scale (BASIS-24), (13) Gambling attitudes scale, and (14) Questionnaire for treatment perception at the Gambling Addicts Club. In order to achieve aforementioned aims, in this longitudinal research beside descriptive statistics, the non-parametric difference testing method was used. More specifically, Wilcoxon's test rank (with computing the size effect) and Spearman's correlation coefficient. Results: The results obtained by the self-evaluation method show the characteristics of pathological gamblers who are in the KLOK psychosocial treatment. Due to the observation of gambling behavior, it has been established that KLOK members mostly satisfy the criteria for moderate and severe addiction, that the most widely used gambling game is sports betting, roulette and slot machines, and that almost everyone gambles on the Internet or mobile apps. The testing of differences has shown that the respondents gamble less during the treatment and, according to the DSM-5 scale, have less pronounced gambling symptoms compared to the period prior to treatment. Furthermore, after six months of treatment there are differences in individual psychosocial functioning variables, exclusively in the domain of gambling cognition, attitudes and cognitive distortions related to gambling, while other variables of psychosocial functioning do not show differences. Six months of treatment also resulted in the difference in the variables of gambling attitudes and depression/functioning in the extracted subsample of participants (N=13) who were at the beginning of the treatment during the research implementation. Additionally, the results show that respondents have a positive perception of the treatment and that a more positive perception is correlated with negative attitudes toward gambling.