"Core" temperature and the proper methods for its assessment and management in cardiac surgical patients with hypothermia continues to be a concern for physicians and nurses. In this study we investigated the relationship between pulmonary artery and urinary bladder temperatures over a 6-hour period during rewarming in 14 (adult) patients in the intensive care unit after cardiopulmonary bypass. Bladder temperatures were 0.1 degree C to 0.2 degree C higher than pulmonary artery temperature with correlation coefficients of 0.94 to 0.99. This relationship continued for most of this period with significant mean differences clustering in the early and late period after admission. The significance of these small differences of temperatures and why the two temperatures reversed at certain time periods needs further investigation because the normal thermal gradients may be altered by the hypermetabolic activities consequent to shivering or iatrogenic overheating.