Young healthy volunteers were treated with physical hyperthermia (baths) in order to investigate changes in blood coagulation. Such therapy is used in the treatment of rheumatic diseases. Single hot baths (mean body temperature 38,2-39,9 degrees C) resulted in a rise of fibrinogen, factors IX and XII, maximal amplitude of the thrombelastogram and hemoglobin and in a decrease of plasminogen. In a series of hypothermic baths no additional changes of coagulation or fibrinolysis could be found. The results suggest that hyperthermia causes a tendency to thrombosis.