['People with severe hemophilia A or B, X-linked bleeding disorders due to decreased blood levels of coagulants, suffer recurrent bleeding into joints and soft tissues. Before clotting factor concentrates were available, most people with severe hemophilia developed crippling musculoskeletal deformities. Clotting factor concentrate prophylaxis aims to preserve joint function by converting severe hemophilia (factor VIII or IX less than 1%) into a clinically milder form of the disease. Prophylaxis has long been used in Sweden, but not universally adopted because of medical, psychosocial, and cost controversies. Use of clotting factor concentrates is the single largest predictor of cost in treating hemophilia.', 'To determine the effectiveness of clotting factor concentrate prophylaxis in the management of people with hemophilia A or B.', "We searched the Cystic Fibrosis and Genetic Disorders Group's Trials Register comprising references from comprehensive electronic database searches and handsearches of journals and abstract books. Reference lists of relevant articles were reviewed. Most recent search: November 2005.", 'Randomized controlled trials (RCTs) evaluating people with severe hemophilia A or B, receiving prophylactic clotting factor concentrates.', 'Two authors independently reviewed studies for eligibility, assessed methodological quality and extracted data.', 'Twenty-nine studies were identified; four studies (including 37 participants) were eligible for inclusion. Three studies evaluated hemophilia A; one showed a decrease in frequency of joint bleeds with prophylaxis compared to placebo (non-physiological dose), with a rate difference (RD) -10.80 (95% confidence interval (CI) -16.33 to -5.27) bleeds per year. The remaining two studies evaluating hemophilia A compared two prophylaxis regimens, one study showed no difference in joint bleed frequency, RD -5.04 (95%CI -17.02 to 6.94) bleeds per year and another failed to demonstrate an advantage of factor VIII dosing based on individual pharmacokinetic data over the standard prophylaxis regimen with RD -0.14 (95% CI -1.34 to 1.05) bleeds per year. The fourth study evaluated hemophilia B and showed fewer joint bleeds with weekly (15 IU/kg) versus bi-weekly (7.5 IU/kg) prophylaxis, RD -3.30 (95% CI -5.50 to - 1.10) bleeds per year.'] Does clotting factor concentrate given to prevent bleeding and bleeding-related complications in people with hemophilia A or B?