The youngest case with spontaneously acquired inhibitor to factor VIII is reported. A 7-year-old girl without any previous disorders presented with a hemarthrosis in the left knee and developed a rapidly expanding hematoma on the right arm, shoulder and chest after an initial infusion of factor VIII concentrate (1,000 units). Circulating anti-factor VIII antibody was detected. Her coagulopathy was successfully managed with an additional massive infusion of factor VIII concentrate (7,500 units) and combination therapy of prednisolone and cyclophosphamide.