Hemophilia B is a hemorrhagic disease characterized by a deficiency of factor Ⅸ activity and a sex-linked recessive mode of inheritance, and is clinically indistinguishable from hemophilia A. Four cases of proven hemophilia B are presented with a brief review of the literature. The first case (M 1-l/2 yr.) was associated with idiopathic thrombocytopenic purpura, and plasma factor Ⅸ level was 0.3% of normal. The second (M 20yr.) and the third (M 24yr.) cases were siblings, with 1.0% and 0.7% of factor Ⅸ activity. Family study of these second and third cases showed that their father had subhemophilic level of factor Ⅸ activity (35%), and their mother, an obligatory carrier, and their sister, a potential carrier, had reduced factor Ⅸ levels, 48% and 35% respectively. The fourth case (M 18 yr.) had 3.2% of factor Ⅸ activity. The authors believe that this is the first report of factor assay-proven cases of hemophilia B in the Korean literature.