Haemophilia A is a rare, inherited X-linked recessive bleeding disorder, resulting from factor VIII deficiency and characterised by intra-articular and intramuscular bleeding.

There are numerous different mutations which cause haemophilia A. Due to differences in the gene involved (and the subsequent resulting protein), patients with haemophilia (PWH) have varying levels of factor VIII clotting activity. Individuals with less than 1 % FVIII clotting activity are classified as having 'severe' haemophilia, those with 1–5 % as 'moderate', and those with between 5–40 % as mild. Most severe haemophilia patients require regular supplementation with intravenous recombinant or plasma derived factor VIII concentrate (prophylaxis).