Continuous monitoring of body temperature during anaesthesia is a widely accepted clinical practice for which a variety of techniques are used. In this study, the accuracy of the deep body thermometer (Coretemp®) was compared with temperatures measured by needle thermocouples. With IRB approval and informed consent, seven ASA physical status I and II patients undergoing otolaryngeal surgery were studied. General anaesthesia and neuromuscular blockade were induced with thiamylal and vecuronium. Anaesthesia was maintained with isoflurane at an end-tidal concentration of 1.0–2.0% and 66% nitrous oxide in oxygen. After induction of general anaesthesia, subcutaneous temperature was measured at the sole of the left foot using a Coretemp®. Additionally, 8-, 18-, and 38-mm-long needle thermocouples were inserted into the the sole of the left foot close to Coretemp® and skin-surface temperature was also recorded adjacent to the needles. The Coretemp® measurement (Tc) correlated best with 18-mm-deep needle temperature (r2 = 0.87). There was also a good correlation between Tc and 38-mm-deep needle temperature (r2 = 0.83). Skin and 8-mm-deep needle temperatures correlated poorly or only moderately with Tc (r2 = 0.67, 0.75, respectively). These results indicate that temperatures measured by Coretemp® well reflect the temperatures at a depth of 18 mm or more from the skin into the foot.