OBJECTIVE: Our objective was to study the effect of the temperature of the anesthetic gas mixture (AGM) on esophageal temperature measurements made in children whose tracheas had been intubated for anesthesia. We also sought to establish the optimal site for the temperature sensor in the esophagus and to find a way to accurately place the sensor. METHODS: Special esophageal temperature probes with thermistors located at 1-cm intervals were used for data collection on a multiplex system. Esophageal temperature measurements were made every 15 minutes for a period of 120 minutes in anesthetized children receiving heated (n = 30) and unheated (n = 30) anesthetic gases. RESULTS: The temperature of the AGM (p < 0.001), the site of measurement (p < 0.001), and the interaction between AGM temperature and site of measurement (p < 0.007) all had a significant effect on esophageal temperature measurements. This effect was greatest at a point 3 cm distal to the level of the tip of the endotracheal tube when AGMs were not heated. CONCLUSION: We conclude that best results are obtained when care is taken to place the thermistor in the lower quarter of the esophagus. (We provide a simple formula for calculating this placement in pediatric patients of varying ages.) Placing the probe by acoustic criteria cannot consistently be relied on to provide good thermometry.