The possibility that dietary-induced thermogenesis may be decreased in obesity has been proposed in recent years. However, the results of human studies so far obtained are conflicting. The present research was undertaken in order to clarify this question. We studied postprandial thermogenesis induced by ingestion of a mixed meal and of a carbohydrate mixture in 15 normal and 12 obese subjects. Blood glucose and plasma insulin levels were measured at the same time. The data obtained have shown that the mean resting metabolic rate (RMR) expressed as a function of body weight3/4, is almost the same in obese as in normal-weight subjects (0.115 +/- 0.018 vs 0.133 +/- 0.021 kj/min/kg3/4, respectively). Moreover, the increment of mixed-meal induced thermogenesis (MM-IT) was 48 +/- 22% in normal and -0.8 +/- 12% in obese subjects, respectively (p less than 0.01). Carbohydrate induced thermogenesis (CHO-IT) appeared slightly higher in normal-weight than in obese subjects (159 +/- 66 vs 98 +/- 46). After carbohydrate ingestion we observed a higher glycemic and insulinemic response in obesity. These results indicate that thermogenesis induced by ingestion of food is reduced in obese subjects; they are also compatible with the idea that insulin resistance could play a role in this phenomenon.