ObjectivesThis study aimed to compare the sensitivity, specificity, receiver operating characteristic (ROC), and area under the curve (AUC) using modified frailty index (mFI), EuroSCORE II, and combined mFI-11 and EuroSCORE II to predict in-hospital mortality and composite morbidities. DesignRetrospective cohort study SettingSongklanagarind Hospital, a tertiary care center in the Southern of Thailand. ParticipantsElderly patients aged ≥ 60 years who underwent elective open-heart surgical procedures on a pump between January 2017 and December 2022 were included. InterventionsROC curves were constructed to evaluate the discriminatory power of EuroSCORE II and mFI-11 for predicting in-hospital mortality and postoperative complications. Measurements and Main ResultsThe actual in-hospital mortality was 2.5 % for all patients. The discriminative accuracy of mFI-11, EuroSCORE II, and combined mFI-11 with EuroSCORE II for predicting in-hospital mortality was good, with respective AUCs of 0.733 (95% CI 0.6157–0.8499), 0.793 (0.6826–0.9026), and 0.78 (0.6686–0.893). The AUC of mFI-11 for predicting postoperative cardiac, respiratory, neurological, and renal complications was 0.558 (95% CI 0.5101–0.6063), 0.606 (0.5542–0.6581), 0.543 (0.4533–0.6337), and 0.652 (0.5859–0.7179), respectively, and that of EuroSCORE II was 0.553 (0.5038–0.6013), 0.631 (0.578–0.6836), 0.619 (0.5306–0.7076) and 0.702 (0.6378–0.7657), respectively. ConclusionFrailty (mFI-11) and EuroSCORE II demonstrated good discrimination in ROC analysis, with EuroSCORE II showing superior predictive accuracy for in-hospital mortality in elderly elective cardiac surgery patients. However, neither score independently predicted mortality in multiple logistic regression, nor did combining them enhance predictive power significantly. Furthermore, both scores were less effective in predicting postoperative complications.