The relation of the forehead-sole deep body temperature difference with the cardiac index (CI), and the relation of that with the systemic vascular resistance index (SVRI) were studied in 61 adult patients after open-heart surgery for 24 hours. A correlation between the deep body temperature difference (X) and CI (Y) was expressed as, Y = -0.21X + 3.42 (r = 0.48), and that between the deep body temperature difference (X) and SVRI (Y), Y = 184X + 808 (r = 0.52). But immediately after the end of operation (n = 61), the correlation coefficients (r) were lower, 0.46 and 0.45, respectively. The deep body temperature difference was 1.8 degrees C on the average in patients whose CI was 2.21.min-1.m-2, and was 4.3 degrees C in patients whose CI was below 2.21.min-1.m-2. Conversely CI was below 2.21.min-1.m-2 in patients whose deep body temperature difference was more than 4 degrees C. We conclude that the measurement of deep body temperature difference is useful as a circulatory monitor, and that a critical level of deep body temperature difference is 4 degrees C.