The thermic effect of medium-chain triglycerides (MCTs) was studied in six lean and six obese young males by evaluating postprandial thermogenesis (PPT) after the ingestion of mixed meals containing either 38 g long-chain triglycerides (LCTs) or 30 g MCTs plus 8 g LCTs. Postabsorptive resting metabolic rate (RMR) was higher (P less than 0.05) in the obese individuals than in the lean ones. PPT, evaluated as 6-h incremental areas above RMR, was greater (P less than 0.05) in both groups after meals containing MCTs. The thermic effect of MCTs was 119.7 +/- 33.9 and 144.7 +/- 48.8 kJ/6 h in the lean and the obese subjects, respectively. The postprandial response of glucose, insulin, and free fatty acids did not depend on the type of oil contained in the meal. Our study shows that PPT is enhanced in both lean and obese subjects when LCTs in a mixed meal are replaced with MCTs.