Oral body temperature was measured in 816 men, 57 and 67 years old, sampled from the general population of Göteborg, Sweden, and 22 physically highly active men, sampled on clinical grounds. The measurements were taken in the morning for 14 months. After adding 0.3 degrees C to the readings to make them comparable with rectal readings, the mean body temperature was 36.8 +/- 0.4 degrees C. There was a seasonal variation with a peak during the winter and a trough during the summer. Body temperature was inversely correlated with height and positively correlated with weight and body fat but not with lean body mass. High physical activity and sensitivity to heat were associated with a higher than average body temperature. Sensitivity to cold was associated with a lower than average body temperature. Smoking prior to the measurements did not appear to affect body temperature.