Lately a number of recombinant Factor VIII (rFVIII) and IX (rFIX) drugs with prolonged half-life have been developed and are about to or even have reached the market. The length of the pro-longed activity of the drug differs between patients and a need for individual tailoring of the patient´s drug administration is obvious. Thrombin generation could prove to be an excellent tool in tailoring the optimal drug administration and monitoring the effect of the drug for each individual.