Forty-two women and 18 men of mean age 54 years had their sub-lingual oral temperature measured hourly from 0700 h to 2300 h. Mean oral temperature (averaged over the 17 readings) was inversely correlated with body mass in the group as a whole (r = -0.44, df = 58, p = 0.0003). The women had significantly higher mean oral temperatures than the men, but the inverse relationship between mean oral temperature and body mass was still significant when the data from the women were analyzed separately (r = -0.37, df = 40, p = 0.013). The results suggest that in humans, mean body temperature is inversely related to body mass, irrespective of gender.