Association of severe haemophilia A and factor V Leiden: report of three cases.

The clinical severity of haemophilia is usually related to residual clotting factor activity. Reports of factor V deficiency or protein C have been described in haemophilic patients and both are involved in the activated protein C pathway. Recently, the poor response to activated protein C due to factor V Leiden (FQ506) has been associated with an increased risk of thrombosis and its incidence in the normal population is given as 2-7%. We describe three cases of severe haemophilia A heterozygous for factor V Leiden and its influence on phenotype.