The levels of factors VIII, II, and VII rise during pregnancy in normal women. In addition, increases in factor VIII levels have been observed in pregnant carriers of hemophilia A and in women affected with von Willebrand’s disease. The influence of pregnancy on factor IX levels is less clear. Consequently, we determined serial factor IX coagulant activities (IX:C) and factor IX antigen levels (IX:Ag) during the pregnancy of an affected carrier of hemophilia B who had baseline values of 13 % for both IX:C and IX:Ag levels. Neither level rose during pregnancy and the patient was treated with plasmapheresis and plasma infusions during the delivery and the postpartum period. Excessive bleeding did not occur.