Prothrombin (factor II) deficiency is a rare autosomal recessive coagulation disorder that occurs in approximately 1 in 1-2 million people. Prothrombin is activated to thrombin, which in turn proteolytically cleaves fibrinogen to fibrin and contributes to forming a stable fibrin clot. The haemostatic level of prothrombin is thought to be between 20 and 40%, and the half-life is approximately 3 days. There are more than 40 known mutations in prothrombin. Both hypoprothrombinemia and dysprothrombinemia have been described. Low prothrombin activity typically prolongs both the activated partial thromboplastin time and prothrombin time. Clinical manifestations are predominantly mucosal or surgical- or trauma-associated bleeding, but joint bleeding and intracranial haemorrhages have been reported. No purified prothrombin products are available for replacement therapy. Both fresh frozen plasma and prothrombin complex concentrates contain prothrombin and may be used for treatment.