A 28 year old pregnant woman was referred for genetic counselling because of a bleeding tendency and a family history of hemophilia. The hemophilia patients had 0.02 units/ml of factor IX activity and a normal concentration of factor IX antigen. In addition they had a prolonged coagulation time with bovine thromboplastin and were therefore cases of hemophilia BM. At the age of six the patient was hospitalized because of prolonged bleeding after a tooth extraction. At the age of 20 and 24 she gave birth to healthy daughters. The first delivery was complicated by a serious bleeding seven days post partum whereas the second delivery was without complications. Factor IX activity when she was three months pregnant was 0.02-0.03 units/ml and the factor IX antigen concentration was normal. Coagulation time with bovine thromboplastin was prolonged. Delivery was again normal, and she had a daughter with carrier values of factor IX. Her mother also had carrier values whereas her father was normal. The patient's hemophilia BM was probably due to extreme Lyonization in a heterozygote.