Background Activated factor XIII (FXIII) cross-links between fibrin monomers, thus increasing the clot stability and resistance to fibrinolysis. Congenital FXIII deficiency causes severe bleeding diathesis. Recently, a common polymorphism of the FXIII A subunit (FXIII Val34Leu) has been identified as a protective factor against both arterial and venous thrombosis. The aim of this study was to investigate the role of FXIII Val34Leu polymorphism in coronary artery thrombosis, especially in young patients. Methods and Results One hundred and thirty patients under than 60 years of age with a history of myocardial infarction (%) and 130 healthy control subjects in the same age group were included to our study. Genomic DNA was extracted from venous blood samples and the polymerase chain reaction method was used to genotype FXIII Val34Leu polymorphism. Coronary risk factors such as obesity, diabetes mellitus, hyperlipidemia and smoking were compared between the groups with chi-square test and logistic regression analysis. The Leu allele frequency was significantly lower in patient group compared to control group (7.69% vs 19.23%, p=0.0001, chi-square). This difference was extremely significant in patients younger than 50 years-old (5.26% vs 19.64%, p<0.0001, chi-square). Conclusion Our findings support the hypothesis that Val34Leu polymorphism in FXIII gene has a protective effect against myocardial infarction. (Circ J 2006; 70: 239 - 242)