© 2021 Ankara Yildirim Beyazit University.Objectives: This study aims to investigate the dynamic thiol-disulfide balance in the early period of patients diagnosed with acute pulmonary thromboembolism in the emergency department. Materials and Methods: Serum thiol-disulfide levels in venous blood samples drawn from 58 patients diagnosed with acute pulmonary embolism in the emergency department were analyzed using the newly developed Erel-Neşelioğlu method, and the results were compared with the measurements of the healthy control group. In the emergency department, the pre-diagnosis of pulmonary embolism was considered with clinical and laboratory findings and confirmed with contrast-enhanced thorax computed tomography angiogram. Results: Mean native thiol and total thiol levels were significantly lower in pulmonary embolism patients compared to the control group (308.53 ± 78; 414.13 ± 50.86; p <0.002; 346.03 ± 81.13; 453.05 ± 52.46 μmol/l, p <0.003 respectively). Disulphide/native thiol and disulphide/total thiol ratios were found to be significantly higher in pulmonary embolism patients compared to the control group (p <0.001). Conclusion: Thiol-disulfide balance is altered oxidatively in patients with pulmonary embolism. This may play a role in the pathogenesis of the disease.