Since December 2001, the Department of Oncology and Medical Physics, Haukeland University Hospital, Norway has been conducting whole body hyperthermia (WBH) studies, treating patients with either ovarian carcinoma or non-Hodgkin lymphomas. Accurate and reliable thermometry instrumentation is important in all types of hyperthermia procedures, particularly in WBH, where the target patient body temperature is 41.8 degrees C. Reliable documentation of side-effects in clinical studies is also dependent on precise temperature monitoring, since in this temperature range even small, but systematic, inaccuracies (0.1-0.2 degrees C) in the temperature monitoring is expected to affect the amount of side effects. Readily available heating and temperature data from previous treatment sessions of the same patient is also valuable for precise temperature control in future treatment sessions. The WBH thermometry system implemented at Haukeland University Hospital is described. It is based on commercially available components, including standard medical thermistor probes, and includes a temperature calibration and verification facility. The thermometry system is accurate, reliable, easy to use, comfortable for the patient and relatively inexpensive. By implementing the Steinhart-Hart polynomial fit to standard medical thermistor probe data, it is shown that the WBH treatment thermometers used can measure the patient body temperatures with a short- and long-term accuracy of +/- 0.01 degrees C.