Liver disease and thromboembolism are the most frequent and severe side effects associated with the use of clotting-factor concentrates in haemophiliacs. Knowledge and careful evaluation of the risk factors related to their development appear to be the most reasonable policy for prevention. themolysis due to significant isoantibody infusion and bleeding associated to abnormal platelet function occur more rarely during intensive replacement therapy with clotting factor concentrates. Finally, abnormalities of renal function have been observed in a number of patients, though they seem to be of minor clinical importance. These complications do not justify withdrawal or limitation of the very effective and life-changing use of concentrates. However, awareness of their occurrence and of their danger requires that specialized hemophilia centres carry out, at frequent intervals, clinical and laboratory testing of the target organs to allow an early detection.