Summary. Treatment of louse-borne relapsing fever was followed by a sudden increase in fibrin degradation productions (FDP) in 6 of 7 patients. The FDP were believed to be the result of fibrinolysis secondary to intravascular clotting in 2 patients who also had transient decreases in plasma fibrinogen concentration and platelet count. In the remaining cases, a mixed form of fibrinolysis with elevated circulating fibrinolytic activity and local lysis secondary to intravascular clotting was presumed to be responsible for the elevated FDP. A heparin infusion in 3 patients did not prevent the increase in FDP following treatment.