© 2021 Turkish Society of Cardiology.Objective: Left ventricular (LV) systolic function measured through LV ejection fraction (LVEF) has prognostic implications in patients with cardiac and non-cardiac conditions. The balance of thiol and disulphide levels reflects oxidative status in the body. In this study, we aimed to investigate the relationship between plasma thiol and disulphide levels, and LVEF calculated by transthoracic echocardiography (TTE).Methods: This retrospective study included 1,048 patients referred for TTE examination and biochemical analyses, including plasma thiol and disulphide levels. After the application of exclusion criteria, the remaining 611 patients were included in the statistical analysis. Patients were classified into two groups, namely normal LVEF (n-LVEF) (n=446) and low LVEF (l-LVEF) (n=165) according to a cut-off level of LVEF 50%. To reduce sample selection bias and adjust for the influence of differences in patient characteristics on LVEF and oxidative status, 1:1 propensity score matching analysis was applied.Results: Propensity score matching analysis yielded 125 patients in both groups with comparable demographics, medications, and blood parameters. Native thiol and total thiol levels were lower in l-LVEF patients than in n-LVEF patients (p<0.001 for both), whereas disulphide levels were higher in l-LVEF group (p=0.008). Native thiol (r=0.384, p<0.001), total thiol (r=0.35, p<0.001), and disulphide levels (r=-0.129, p=0.004) significantly correlated with LVEF. Conclusion: Plasma thiol levels decrease and disulphide levels increase suggesting the presence of oxidative stress in patients with l-LVEF. Significant correlation between oxidative stress and LVEF sheds light about the possible pathogenetic role of thiol and disulphide in heart failure.