Prothrombin (factor II) deficiency was first described in 1947 by Quicket al., although the first prothrombin abnormality was reported in 1969 by Shapiroet al.The condition is still considered very rare. In spite of its rarity, the defect has allowed important improvements in our understanding of both congenital and acquired prothrombin deficiencies. The diagnosis of prothrombin deficiency or abnormality can be made using a combination of clotting, chromogenic and immunological assays. In cases of true deficiency, a parallel decrease in all these assays is observed, regardless of the activating agent. If discrepancies among the clotting assays are noted, particularly using viper venoms, a dysprothrombinemia should be suspected. Usually, activity levels less than 10% of normal are found in homozygotes, and between 40 and 60% in heterozygotes. Factor II levels in congenital dysprothrombinemias are more variable since one may encounter homozygotes, heterozygotes and compound heterozygotes between a heterozygous abnormality and heterozygous ‘true’ deficiency or between two distinct abnormalities. Usually the levels of factor II vary between 1 and 50% of normal. Antigen levels in congenital dysprothrombinemias will be normal, near normal or slightly decreased but always higher than the clotting counterpart. Cases with a parallel decrease in prothrombin activity and antigen should not be considered as examples of hypoprothrombinemia. The gene involved in the synthesis of prothrombin is located in chromosome 11. It is composed of 10 exons and 8 introns. Molecular biology studies have discovered several point mutations in some of the dysprothrombinemias. Bleeding manifestations may be severe in homozygous ‘true’ deficiency and may be more variable in dysprothrombinemias. Heterozygotes are usually asymptomatic. Prognosis is variable and generally in agreement with the prothrombin activity level. In homozygous true deficiency, hemarthroses and intracranial bleeding have been described. Substitution therapy is based on the administration of prothrombin complex concentrates or of plasma. The long half-life of prothrombin injected, about 70 h, allows the achievement of hemostatically effective levels (about 50% of normal) without difficulty.