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Background and Objectives: This study aimed to evaluate the impact of body mass index (BMI) on post- operative outcomes in patients undergoing aortic arch repair with the frozen elephant trunk technique (FET). Materials and Methods: A total of 387 patients who underwent an FET procedure between 04/2014 and 11/2024 were retrospectively analyzed. Patients were divided into four groups according to BMI: underweight (BMI < 18.5, n = 12) normal weight (BMI: 18.5 to <25, n = 150), overweight (BMI: 25 to <30, n = 154), and obese (BMI: ≥30, n = 71). Patient characteristics and clinical outcomes were compared across groups. Multivariable Cox regression, interaction analysis, and restricted cubic spline modelling were performed using R (Version 4.4.3). Results: Interaction analysis revealed BMI-dependent effect modification for several predictors. Insulin-dependent diabetes mellitus was associated with increased mortality only in patients with BMI < 25 kg/m² (interaction p = 0.003). Transfusion of packed red blood cells (PRBCs) also showed a significant interaction with BMI (p = 0.016), with a stronger effect in patients with BMI < 25 kg/m², although significant in both strata. Moreover, cross-clamp time demonstrated a BMI-dependent interaction (p = 0.047), with numerically higher mortality hazards in overweight patients (BMI > 25 kg/m²), but without statistically significant subgroup effects. Spline analysis indicated a non-linear, threshold-based association between overall mortality and BMI but does not reach statistical significance. Kaplan-Meier analysis showed no significant difference in 5-year survival among BMI categories. Conclusions: BMI should not be used as a primary risk stratification tool for survival after an FET procedure. Rather, attention should be paid to comorbid conditions and intraoperative factors that interact with BMI. For patients with lower BMI (<25 kg/m²), optimizing glycemic control and minimizing transfusion may improve outcomes. Data suggests that a reduction in cross-clamp time may be particularly beneficial in patients with higher BMI (>25 kg/m²). Future studies should aim to clarify the impact of BMI on outcomes after FET, particularly in the context of patient selection and perioperative optimization strategies.