Clotting factor replacement therapy has proven a highly effective means of treating haemophilia A and B. But treatment involves frequent and lifelong infusion of factor concentrates and is generally prophylactic rather than curative. It is also extremely expensive, associated with inhibitor formation and does not fully abolish the potential for spontaneous bleeding. Gene therapy offers a potential cure for haemophilia, with the possible continuous expression of a clotting factor gene following the administration of a viral vector carrying the appropriate gene. Recent clinical trials of gene therapy for haemophilia have proven positive in selected patients and new studies are underway.