The outpatient clinic provides a special institutional service for psychiatric patients who need treatment in an environment similar to a hospital but allows them to return home and integrate into everyday life after therapy. This form of treatment is suitable for patients who do not require hospitalization, but who need a more intensive therapeutic program that includes active treatment. Objective: To examine the differences in mature, immature and neurotic defense mechanisms before and after treatment in the Outpatient Clinic of Psychiatry. Respondents and methods: The research was conducted in 2023. Ninety-two patients treated in the Outpatient Clinic of Psychiatry participated in the research, of which 18 (56.5%) were female and 74 (80.4%) were male, and the mean age of the subjects was M=52.45 (range from 27 to 74; SD=10.34). Demographic Questionnaire and Defense Style Questionnaire (DSQ-40) were used in the research. Results: Overall, looking at defense mechanisms, the results showed that the neurotic defense mechanisms were significantly lower after treatment (t-test; p=0.039). Individually looking at mature defense mechanisms, after treatment the subjects used sublimation significantly less (ttest; p=0.049) while they used humor significantly more (t-test; p=0.008). In the case of neurotic defense mechanisms, after treatment, subjects used pseudo-altruism (t-test; p=0.038) and idealization (t-test; p=0.003) significantly less, while they used reactive formation significantly more t-test; p=0.035); while with immature defense mechanisms after treatment, they used acting out (t-test; p=0.006), denial (t-test; p=0.012) and splitting (t-test; p=0.014) significantly less, while they used passive aggression significantly more (t-test; p=0.002). Conclusion: Despite the relatively short duration of treatment, these findings suggest that even relatively short-term therapeutic procedures can have a significant impact on reducing maladaptive defense mechanisms. These results emphasize the importance of therapeutic interventions in promoting adaptive defense mechanisms and reducing dysfunctional defense strategies. Although further research is needed to understand the long-term effects of therapy on defense mechanisms, these preliminary findings suggest that therapists can successfully intervene in reducing unhealthy defense patterns in their patients even in a relatively short period of time.