Elective orthopaedic surgery was performed in a boy with hemophilia A associated with a low titer antihemophilic globulin (Factor VIII). A few weeks later an emergency knee disarticulation had to be performed at a time when the inhibitor titer was elevated to over 1:400. Continuous infusion of massive doses of human antihemophilic factor concentrates during surgery and for several days postoperatively was used to control bleeding. Severe hemolytic anemia due to anti-B in the antihemophilic factor concentrates was detected and was promptly controlled with transfusion of Type 0 Rh negative red blood cells. Hyperfibrinogenemia without clinical complication was observed after the infusion of massive doses of antihemophilic globulin concentrates. Careful evaluation for the presence of a circulating inhibitor to antihemophilic factor prior to undertaking surgery is emphasized.