OBJECTIVES: Regular prophylactic replacement therapy has been proved to be effective in reducing the bleeding rate and incidence and/or severity of haemophilic arthropathy in children with severe haemophilia. No study is so far available that evaluates efficacy of prophylaxis in adolescents and adults with haemophilia A. This study is aimed to provide cost effectiveness evaluation of prophylaxis in adult with haemophilia A. METHODS: A prospective, open study was designed. Patients with haemophilia A aged 18 years or more, with frequent bleeding episodes, switching from on-demand treatment to prophylaxis, have been enrolled. All patients were treated with a recombinant B-domain-deleted factor VIII concentrates for all the 6-month study period. Bleeding event rate and FVIII concentrate consumption (that represent 99% of total costs) have been evaluated over on-demand treatment time period (ODT), 6 months before enrolment, and the prophylaxis time period (PT), 6 months after enrolment. Medical costs have been quantified adopting the perspective of the third party payer, i.e. National Health Service (NHS). RESULTS: Ten patients with a median age of 34.0 years (23-58 years) were enrolled. Patients reported a mean of 3.6 during ODT (median=2.0, 1.5-15) vs. 0.51 events/patient/month during PT (median=0.16, 0-1.7). Clotting factor mean consumption was 22,010 during ODT (median=17,750 IU, 4,500-50,000) and 28,817 IU/patient/month during PT (median=28,333 IU, 21,333-38,333). Mean cost of concentrates in ODT was 10,911 (median=€9607, 2,193-23,500) while during PT was 19,883 €/patient/month (median=19,550, 14,720-26,450). The mean cost to treat one bleeding in ODT was 3031. The incremental cost-effectiveness ratio, i.e. the cost for bleed avoided, was €2803. CONCLUSIONS: These findings showed that prophylaxis in adult patients with haemophilia is effective in reducing the bleeding rate. Despite the higher cost of prophylactic treatment compared to on-demand treatment prophylaxis showed to have a good cost-effectiveness ratio.