Haemophilia B is a disease caused by a deficiency in factor IX (anti-haemophilia factor B) that affects around one in 30 000 male infants at birth.
The disease is inherited as an X-linked recessive trait and thus occurs in males and very rarely in homozygous females. Heterozygous females for the disease are known as carriers.

Classification of the disease is based on the severity of factor IX deficiency, the actual cause of the haemorrhagic signs.

Haemorrhagic episodes occur primarily in joints and muscles.