The temperature gradient between the ventral surface of the first toe and the ambient temperature was measured and compared with established hemodynamic measurements in 71 critically ill patients. Thirty-two patients had acute myocardial infarctions, 21 patients had primary bacteremia and 18 patients had primary hypovolemia which followed acute blood loss. The temperature gradient served as a more predictable indicator of survival or fatality than either arterial pressure or cardiac index in each group of patients. Patients who improved after treatment and survived had increases in the toe minus ambient temperature gradient to more than 4 degrees C., whereas a gradient of less than 3 degrees over an interval of 12 hours was typically observed in patients who subsequently died. These observations indicate that the toe minus ambient temperature gradient provides a valuable, inexpensive and noninvasive monitor of tissue perfusion in critically ill patients.