Seven patients with classic hemophilia A had alteration of primary hemostasis after treatment with lyophilized antihemophilic globulin (LAHG). The following test results, which were normal before treatment, became abnormal after treatment: bleeding time, bleeding intensity, and platelet adhesiveness. In two patients, the fibrin‐fibrinogen‐degradation products increased to more than 40 μg/ml. In three patients, the bleeding symptoms became worse with LAHG therapy although no inhibitor against Factor VIII was demonstrated. In one of these patients, the bleedings symptoms disappeared when the use of LAHG was discontinued and prednisone was given; at the same time, the altered primary hemostasis returned to normal. In the remaining two patients, prednisone did not have any effect. In these two patients, however, the bleeding stopped, and the bleeding time became normal immediately after freshly prepared blood‐group compatible cryoprecipitate was