Haemophilia B is extremely rare in females and so far 20 cases have been reported. A 9-year-old girl with severe haemophilia symptoms is described, who shows a very low level of factor IX activity (1.5%) and antigen (less than 10%), normal XX female karyotype and negative family history of bleeding tendency or consanguinity. This case is probably the fourth report of sporadic female haemophilia B without chromosomal aberration and the first one with quantitative analysis by immunoradiometric assay and enzyme-linked immunosorbent assay of factor IX antigen. This very low factor IX level and severe bleeding tendency of the patient would be consistent with the homozygous state, but since a double mutation is extremely rare the patient's laboratory findings can be more easily explained by extreme lyonization of the normal X-chromosome of a heterozygous carrier.