Coagulation studies were performed on 10 patients after the slow induction of hypothermia and later during surgery and transfusion. In contrast to what has been reported in studies on dogs, hypothermia alone in man was not associated with a fall in concentration of clotting factors as determined by platelet counts, thromboplastin generation, prothrombin consumption, prothrombin times, and specific assays of prothrombin, proconvertin and accelerator globulin activity and fibrinogen levels. Excessive hemorrhage was not encountered during or after surgery. Moderate disturbances in coagulation with surgery and transfusion during hypothermia were in most regards similar to those observed with surgery and transfusions at normal body temperature. The one difference was a decrease in prothrombin consumption. The cause of this abnormality is obscure.