Recently, numerous studies have suggested an association between factor XII (FXII) deficiency and recurrent pregnancy losses, and between autoantibodies to FXII and recurrent pregnancy losses. Autoantibodies to FXII rather than FXII deficiency may be a risk factor for recurrent pregnancy losses. To know the pathogenesis of autoantibodies to FXII, epitope mapping study was done. Seventeen anti-FXII antibody positive recurrent pregnancy loss patients were chosen for this study. We used synthetic peptides in inhibition and direct binding studies to identify the antigenic binding site of autoantibodies to FXII. Among plasmas from 17 recurrent pregnancy loss patients who were positive for autoantibodies to FXII, 13 patients (76.5%) recognized amino acids 1-30, the amino-terminal heavy chain region that is known as factor XII binding site to platelet glycoprotein Ibalpha.