Hypoprothrombinemia is a rare hereditary coagulation defect characterized by low levels of biologically active prothrombin. In this paper we report the laboratory and genetic analysis of a patient with a severe hypoprothrombinemia and some of her relatives. Laboratory analysis showed very low levels of prothrombin antigen. Molecular analysis of the prothrombin genes of the patient resulted in the identification of two novel sequence variations in heterozygous state, a 20079 G to A transition, which predicts a Trp 569→Stop mutation, and a 1261C→G change within intron B near the acceptor splice site. A cosegregation of prothrombin deficiency in family members with the two genetic defects was observed.