The thermic response of five lean and five obese subjects was measured by indirect calorimetry before, and for 157.5 min after a meal of protein, carbohydrate or fat, each of which provided 1.25 MJ. The change in plasma glucose, insulin and (in the case of the carbohydrate meal) the rate of exogenous glucose oxidation was also measured. There was no significant difference between the lean and obese groups in the magnitude of the thermic response to any of the three meals. In both weight groups the response was largest and most prolonged after the protein meal (P less than 0.01). The obese group showed a higher concentration of fasting plasma insulin (P less than 0.01) and a larger increase in plasma glucose (P less than 0.05) after the carbohydrate meal, but there was no significant difference in the oxidation of exogenous glucose when compared with the lean group. Previous studies on dietary-induced thermogenesis in lean and obese subjects have given conflicting results. In general reports of decreased thermogenesis in obese subjects are characterized by either (a) high pre-meal metabolic rates in the obese group, especially in diabetic subjects, or (b) a group classified as 'normal' who have been selected for their high thermogenic capacity.