Samples for above tests will be collected that afternoon, the following morning and the following afternoon. The samples can be taken at any convenient time but the exact time (hours and minutes) must be recorded. Factor VIII levels will be measured. The patient's weight (kg), amount and exact time of the last 3 factor VIII infusions, measured factor VIII levels and exact times of when the samples were taken will be communicated to the centre in Uppsala and the pharmacokinetic parameters established using a Bayesian procedure based on a population model.