© 2019, Tehran Heart Center. All rights reserved.Background: The aim of this study was to investigate the ability of a new index, namely the left ventricular internal dimension at end-diastole/mitral valve E-point septal separation (LVIDd/EPSS), to predict the left ventricular (LV) systolic function and to compare its performance with that of the EPSS index and to investigate the correlation between the LVIDd/EPSS and the left ventricular ejection fraction (LVEF). Methods: The current study recruited 142 patients who presented to the Cardiology Clinic of Sakarya University Education and Research Hospital and were followed for heart failure (HF).M-mode measurements of the EPSS and the LVIDd were recorded in the parasternal long-axis view. Results: Totally, 142 HF patients with midrange ejection fraction (HFmrEF) and reduced ejection fraction (HFrEF) were enrolled in the study. There was a significantly correlation both between the EF and the EPSS and between the EF and the LVIDd/EPSS (P<0.001). In both HFmrEF and HFrEF groups, the correlation between the LVIDd/EPSS and the EF was more significant than was the correlation between the EPSS and the EF (P<0.001). The results of the linear regression analysis indicated that the LVIDd/EPSS was an independent predictor of the HFmrEF and the HFrEF (P<0.001). In the patients with EPSS≤12, there was a significant association between the EF and the LVIDd/EPSS (P<0.001) but not between the EF and the EPSS(P>0.05). The receiver operating characteristic curve analysis showed that the LVIDd/EPSS predicted advanced HF with 87% sensitivity and 72% specificity, using a cutoff value of 3.35,and it predicted the HFrEF (EF<40%) with 84% sensitivity and 81% specificity, using a cutoff value of 3.75. Conclusion: The LVIDd/EPSS may allow certain clinicians, especially beginners and emergency department physicians, to assess the LVEF when other methods are not available or questionable.