Objective: This study aims to evaluate the effectiveness and patient satisfaction of Group Schema Therapy-Day Treatment (GST-d) in treating individuals with cluster B/C personality disorders. Method: The study utilized a naturalistic design with 39 patients undergoing 34 weeks of combined group and individual schema therapy. This included weekly full-day GST-d sessions, 15 individual sessions, and two group follow-ups. Results: Significant reductions were observed in psychological symptoms, personality issues, and dysfunctional schemas, with effect sizes (Cohen d’s) ranging from .77 to .87. Healthy modes showed a smaller effect size of .24. The dropout rate was 17.9%, but overall patient satisfaction remained high. Conclusions: The findings suggest the potential effectiveness of GST-d in treating personality disorders. Despite some limitations, the study recommends further research with a larger sample and randomized control trials to better understand the relationship between patient satisfaction and treatment outcome, as well as cost-effectiveness.