Klinefelter syndrome (KS) is a common sex chromosome-related abnormality seen among men. KS negatively affects spermatogenesis and testosterone production. It increases the risk of thrombosis but its molecular mechanism has not been well described yet. Elevated PAI-1 is a risk factor for thrombosis. The rs1799889 polymorphism located in the promoter region of the PAI-1 gene was detected in patients with deep venous thrombosis. In this study, the PAI-1 gene variant and its plasma levels in KS patients were examined. Forty-one KS patients (47, XXY) and 50 age-matched healthy controls participated. DNA was isolated from peripheral blood and a real-time PCR method was used to detect known SNPs in the PAI-1 gene. In addition, PAI-1 plasma levels were measured by using ELISA method. There was no significant difference between PAI-1 gene polymorphisms of KS patients and controls (p > .05). The significant difference was observed in PAI-1 plasma levels between two groups (high PAI-1 plasma level in KS patients compared to controls). The patients' group mean was 55.13 and control group mean in PAI-1 level was 29.89 ng/ml (p = .020). Clinical features related to thromboembolism especially varicose veins were detected in KS patients frequently (p = .04). These results suggest that thromboembolism related to clinical features is seen more frequently in cases with KS, but it may not be dependent only on the PAI-1 gene polymorphism structure.