The effect of local hyperthermia on subcutaneous tissue oxygen tension (Psqo2) and perfusion was investigated in eight patients (13 trials) using a subcutaneously implanted oxygen tonometer. Application of heat increased subcutaneous tissue temperature and Psqo2. Mean Psqo2 during oxygen breathing rose by 39.5 mm Hg, an 80% increase over the average baseline Psqo2. The corresponding mean subcutaneous temperature increased 4.0 degrees C. A significant linear correlation was found between the change in Psqo2 and subcutaneous temperature. There was an average threefold increase in local perfusion estimated by using the Fick principle. The data reaffirm the value of local hyperthermia in treating contaminated wounds and suggest a mechanism for its ability to ameliorate infections. The mechanism implies that local heat may have prophylactic value as well.