The aim of this study is to determine the serum endocan levels in patients with pulmonary thromboembolism (PTE) and investigate whether a relationship exists between serum endocan levels and the disease severity.The study included 85 patients with acute PTE and 40 healthy control subjects. The patients with PTE were divided into 3 groups at admission as "high-risk", "intermediate-risk" and "low-risk", considering the guidelines of the European Society of Cardiology. Serum endocan levels in all participants' blood samples were measured.The mean serum endocan levels were significantly higher in the PTE group, compared to the control subjects (p<0.001). Serum endocan levels were significantly higher in the "high-risk" group when compared with patients in the "low-risk" and "intermediate-risk" groups (p<0.001 and p<0.01, respectively). Similarly, serum endocan levels were higher in the "intermediate-risk" group compared to those in the "low-risk" group (p<0.001). There was a negative correlation between serum endocan levels and partial oxygen pressure (r=-0.262, p=0.016), whereas a positive correlation was found between the serum endocan levels and systolic pulmonary arterial pressure (r=0.296, p=0.006). Additionally, endocan had an area under the curve (AUC) in the receiver operating characteristic (ROC) curve of 0.837 (0.768 - 0.907; 95% CI; p<0.001) and cut-off value was 194.5 pg/mL (sensitivity 80%, specificity 72.5%).Serum endocan levels were higher and related to the severity of the disease in PTE patients. Additionally, endocan could be an indicator to be used in the diagnosis of PTE and in the prediction of the disease severity.