This research study was undertaken to determine the relationship between pulmonary artery (PA) blood temperature, rectal temperature and axillary temperature for adult patients admitted to an intensive care unit (ICU). 31 adults had their temperatures monitored. Simultaneous recordings of PA blood temperature, rectal temperature and axillary temperature were taken every 4h for up to 7 days. The mean difference between rectal and axillary temperature for all 31 patients was 0.32 degree C. Of the 16 patients who had their PA blood temperature monitored mean rectal temperature was 0.1 degree C above PA blood temperature and mean axillary temperature was 0.19 degree C below PA blood temperature. Very high statistical correlations were obtained which demonstrate the strength of the relationships between the three sites (rectal-axillary temperature difference R = 0.97; PA blood-rectal temperature difference R = 0.99; PA blood-axilla temperature difference R = 0.97). The linear relationship between the three sites studied was not found to be affected by age, gender, number of temperature samples taken, post/non-operative admission or peripheral temperature.