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The intrinsic clotting, the kinin generating and the fibrinolytic systems were investigated in 10 patients with hereditary angioedema (HAE), 10 patients with chronic urticaria and 18 healthy volunteers. In spite of the fact that patients suffering from HAE severely lack C1 INH, neither the intrinsic coagulation nor the fibrinolytic systems are impaired. There was a slight decrease of plasma kallikrein--as already known--and moreover a greater decrease in HMW-kininogen, and increase in Factor XII levels. Furthermore, activation of pre-kallikrein was delayed in these patients. These findings make it apparent that lowered HMW-kininogen levels compensate the lack of C1 INH, thus preventing an enhanced activation of the intrinsic clotting and the fibrinolytic systems.