Introduction: High shock impedance is associated with conversion failure among patients with subcutaneous implantable cardioverter defibrillators (S-ICD). Currently, there is no preoperative assessment method for predicting high shock impedance. This study aimed to examine the efficacy of chest computed tomography (CT) as a preoperative evaluation tool to assess the shock impedance of S-ICDs. Methods and Results: The amount of adipose tissue adjacent to the device and the anteroposterior diameter at the basal heart region were measured preoperatively using a chest CT. We examined the correlation between these measurements and shock impedance at the conversion test. We enrolled 43 patients with S-ICDs (age: 54±15 years, body mass index: 23±4 kg/m ², PRAETORIAN score: 30–270 points, amount of adipose tissue 1250±716 cc), who underwent intraoperative conversion tests by inducing ventricular fibrillation, which was terminated with a 65 J shock. A strong concordance correlation coefficient was observed between the shock impedance and the amount of adipose tissue (r=0.616, p < 0.01) and anteroposterior diameter (r=0.645, p < 0.01). Moreover, these were identified as independent predictive factors of shock impedance (amount of adipose tissue: β=0.439, p = 0.009; anteroposterior diameter: β=0.344, p = 0.038) in the stepwise multiple regression analysis. Conclusions: Preoperative CT-measured amount of adipose tissue and basal heart anteroposterior diameter are independent predictors of shock impedance. These may provide better accuracy in identifying high shock impedance in patients with S-ICDs.