The relationships of the forehead-sole deep body temperature difference with the cardiac index (CI), and with the systemic vascular resistance index (SVRI) were studied in 10 pediatric patients (TOF 5, ASD 3, VSD 2) for 24 hours after open-heart surgery. A correlation between the deep body temperature difference (X) and CI (Y) was expressed as, Y = -0.49X + 4.51 (r = 0.72), and that between the deep body temperature difference (X) and SVRI (Z) as Z = 152X + 1039 (r = 0.62). The deep body temperature difference was 1.0 degrees C on the average in patients whose CI was more than 3.0 l.min-1.m-2, and was 3.3 degrees C in patients whose CI was less than 3.0 l.min-1.m-2. Conversely CI was less than 3.0 l.min-1.m-2 in patients whose deep body temperature difference was more than 3.0 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 3 degrees C in children.