International Immune Tolerance Study

Inclusion Criteria: * Severe hemophilia A (FVIII level \<1%). * A maximum historical inhibitor titer of between 5 BU and 200 BU that must be confirmed once prior to the beginning of ITI. * The inhibitor titer should be \<10 BU at the start of ITI, confirmed once. * The inhibitor must be present for \<24 months when ITI begins. * Maximum age of 7 at the start of ITI. * Willingness to comply with the protocol. Exclusion Criteria: * Moderate or mild hemophilia A (FVIII level \>1%). * Spontaneous disappearance of the inhibitor prior to ITI. * Historical maximum inhibitor titer \<5 BU or \> 200 BU before starting ITI. * Inhibitor titer \> 10 BU at the start of ITI. * Inhibitor present for more than 24 months before starting ITI. * Systemic immunomodulatory drug therapy during immune tolerance e.g. corticosteroids (\< 5 days every 2 months maximum dose 2 mg/kg or 60 mg/day), azathioprine, cyclophosphamide, high-dose immunoglobulin or the use of a protein A column or plasmapheresis. * Age \> 7 years at the start of ITI. * Inability or unwillingness to comply with the protocol. * Previous attempt at ITI.