Objectives: This study aims to investigate the dynamic thiol-disulfide balance in the early period of patientsdiagnosed with acute pulmonary thromboembolism in the emergency department. Materials and Methods: Serum thiol-disulfide levels in venous blood samples drawn from 58 patientsdiagnosed with acute pulmonary embolism in the emergency department were analyzed using the newlydeveloped Erel-Neşelioğlu method, and the results were compared with the measurements of the healthycontrol group. In the emergency department, the pre-diagnosis of pulmonary embolism was considered withclinical and laboratory findings and confirmed with contrast-enhanced thorax computed tomographyangiogram. Results: Mean native thiol and total thiol levels were significantly lower in pulmonary embolism patientscompared 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 significantlyhigher 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 playa role in the pathogenesis of the disease.