Factor XII (F.XII, Hageman factor) is one of the contact system factors which initiates an intrinsic coagulation
pathway. But its definite role is still unclear, because many cases of severe F.XII deficiency experience thrombotic events
instead of a bleeding problem. Moreover, most of them are asymptomatic. There have only been a few reports of F.XII
deficiency in Thailand. The author reports two cases of congenital F. XII deficiency in Thai children.

Keywords: Prolonged aPTT, Factor XII deficiency, Thrombosis