A systemic reappraisal of the thermic effect of food was done in lean and obese males randomly fed mixed meals containing 0, 8, 16, 24, and 32 kcal/kg fat-free mass. Densitometric analysis was used to measure body composition. Preprandial and postprandial energy expenditures were measured by indirect calorimetry. The data show that the thermic effect of food was linearly correlated with caloric intake, and that the magnitude and duration of augmented postprandial thermogenesis increased linearly with caloric consumption. Postprandial energy expenditures over resting metabolic requirements were indistinguishable when comparing lean and obese men for a given caloric intake. Individuals, however, had distinct and consistent thermic responses to progressively greater caloric challenges. These unique thermic profiles to food ingestion were also independent of leanness or obesity. We conclude that the thermic effect of food increases linearly with caloric intake, and is independent of leanness and obesity.