Female carriers of hemophilia are usually asymptomatic. Female hemophilia B or symptomatic carriers of hemophilia B are extremely rare and so far only 16 cases have been reported.We report a girl with hemorrhagic diathesis in a family of mild hemophilia B, who was suspected female hemophilia B. The patient was referred to KCMC at the age of 8 years because of easy bruising, which had first been noted at the age of 3 years. The family history showed that father and brother were mild hemophilia B, whose factor IX activity was 36% (4.5%) and 15%, and that her material paternal grandmother, uncle and her cousin had mild bleeding tendency. Mother had no bleeding tendency with normal activity of factor IX, but she was suspected a carrier of hemophilia B because her son was mild hemophilia B. The coagulation findings were as follows: Prothrombin time; 11.9sec., Kaolin activated PTT; 55-63.5sec., Factor IX activity; 4-16%, Factor II, V, VII, VIII, X, XI, XII activity and Fibrinogen level were normal.Platelet function tests: Bleeding time; 3min. 30sec., Platelet retention test; 24%, and Platelet aggregation tests (ADP, Collagen and Ristocetin induced) were normal. The karyotype was found to be 46, XX, although sex chromatin was 4%.If mother is a silent carrier, it is possible for this patient to be female hemophilia B. If mother is not a carrier, we must postulate hemophilia B transmitted by autosomal dominant pattern of inheritance or new disease with low activity of factor IX.We will study on immunological determination of factor IX and more precise family history.