At 3, 6 and 9 months (+/- 2 weeks) of starting the pharmacokinetic tailored treatment phase (i.e. at the end of 9, 12 and 15 months (+/- 2 weeks) from starting the study), a doctor will review the patient and the diary and make appropriate changes to treatment according to the Phase 3 dose escalation protocol. At each of these visits participants will have their weight and factor VIII levels measured As before there will be no wash out. The patient's routine prophylactic infusion will be given in the morning and the exact time and dose recorded. The date, exact time and dose of the previous 2 prophylactic doses must also be accurately known and recorded.