© 2021, Arquivos Brasileiros de Cardiologia. All rights reserved.Background: Ventricular arrhythmias (VAs) are the main cause of in-hospital mortality and morbidity in acute coronary syndrome (ACS) patients and its relationship with thiol is not known. Objective: To investigate the relationship between plasma thiol levels and troponin levels in patients with ACS and to estimate in-hospital VA development during hospital stay. Method: The study included 231 consecutive ST-segment elevation ACS (STE-ACS) and non-ST-segment elevation ACS (NSTE-ACS) patients. After application of exclusion criteria, 191 patients were included in the statistical analysis. Patients were classified into two groups: STE-ACS group (n=94) and NSTE-ACS group (n=97). Plasma thiol, disulphide and troponin levels were measured and troponin-to-native thiol ratio (TNTR) was calculated. A two-sided p value of less than 0.05 was considered to be statistically significant. Results: Plasma native thiol, total thiol, disulphide and their ratios were similar between the groups. TNTR was significantly higher in the STE-ACS group compared to the NSTE-ACS group. Troponin and thiol levels correlated negatively and significantly. Native thiol was found to be an independent predictor of VA development in STE-ACS patients and in all ACS patients. TNTR was found to be an independent predictor of VA development in NSTE-ACS patients and in all ACS patients. Conclusion: Plasma thiol levels can be used to identify ACS patients at high risk for in-hospital VA development. Correlation between troponin and thiol levels may suggest that thiols may be an important marker for diagnosis and prognosis of ACS with the help of future studies.