A rare case of blood coagulation disorder, congenital factor XII deficiency, detected in a patient with mandibular osteomyelitis is presented. Routine laboratory tests showed prolonged clotting and activated partial thromboplastin time. Detailed investigations for the intrinsic and extrinsic pathways of coagulation, fibrinolytic system, kinin-kallikrein system, and complement system were performed because factor XII is known as an activator of these systems. No hemorrhagic or thromboembolic complication occurred during and after surgery. Substitution therapy with fresh frozen plasma was not necessary. Magnetic resonance imaging and electrocardiography were used to examine the possible occurrence of postoperative cerebral hemorrhage or myocardial infarction.