The amount of factor VIII required to maintain this trough level varies markedly between patients because their factor VIII half lives are different. This study will assess the role of regular pharmacokinetic (PK)monitoring and dose adjusted factor VIII to establish whether this is a more cost effective way of giving treatment and whether it is feasible in routine clinical practice. Patients will be treated for 6 months with their standard factor VIII regimen and followed up to establish their bleed frequency. They will then receive pharmacokinetic adjusted factor VIII to maintain a trough above 1.5% for a year and their bleed rate compared to standard treatment. If they have increased break through bleeds their factor VIII will be increased to maintain a trough of 3%.