Haemophilia A is caused by a missing or dysfunctional clotting factor VIIl. Although haemophilia A is usually inherited, a spontaneous mutation of the person’s own genes is responsible for about 30% of cases. In Switzerland, approximately 600 people are affected by haemophilia A of varying degrees of severity. Haemophilia A is diagnosed by measuring the factor VIII activity in the blood. Depending on the factor VIII activity measured, the severity of haemophilia A is categorised as mild, moderate or severe. About one-third of patients suffer from the most severe form of this disease. Patients with haemophilia A have prolonged bleeding after injuries, operations, and dental treatment. In severe cases, there may also be spontaneous bleeding, that is to say, bleeding for no obvious reason. Severe complications may arise from bleeding into the joints, muscles, brain or other internal organs. Mild cases of haemophilia A may remain undetected until the person has an operation or a serious accident. Treatment of haemophilia A is extremely effective. With the correct treatment and medical care, patients with haemophilia A can lead a completely healthy life.