To investigate the possibility that overprediction of basal metabolic rate (BMR) of tropical populations is related to differences in fatness between tropical and temperate populations, 32 Mexican men aged 18-40 years underwent measurements of BMR and body fat content. The men were divided into four body mass index (BMI) groups (< 20, 20-25, 25-30 and 30-40). The mean weight was 78 (range 52-119) kg and the mean height was 1.75 (range 1.61-1.89) m. BMR was measured in the subjects after an overnight stay in a residential unit by a Deltatrac ventilated hood indirect calorimetry system. Body fat content was determined by 2H2O dilution. The relationship between BMR and body weight was analysed by linear regression and the equations derived were compared to the values predicted using the FAO/WHO/UNU (1985) equations. The FAO/WHO/UNU equations overestimated measured BMR by 9.8%, 9.6%, 7.8% and 5.5% in the four groups. The overall difference was 8.2%, which was statistically significant (P < 0.001). The intercepts of the two equations were significantly different (P < 0.001) by 532 kJ/day, but there was no significant difference between the slopes of the two regression equations. There was no significant difference between BMR/kg fat-free mass in the four BMI groups. The results therefore do not support the possibility that the overprediction of BMR in tropical populations by international prediction equations based on body weight is accounted for by differences in fatness between tropical and temperate populations.