Meal-induced thermogenesis is reported to be reduced in obese patients and the purpose of this study was to find out whether thermogenesis after a mixed meal differs between previously obese (post-obese) subjects and non-obese controls. Nine post-obese patients (body mass index 27 +/- 1) and nine age- and sex-matched non-obese volunteers (body mass index 22 +/- 1) were studied with continuous indirect calorimetry in the basal state and after the ingestion of a standardized test meal. Following treatment with vertical banded gastroplasty for the obesity, the weight of the patients had decreased by an average of 44 +/- 5 kg. Basal oxygen uptake and energy expenditure did not differ between the post-obese and the controls. After the meal, energy expenditure increased rapidly, becoming relatively constant after 60 min; the average increase above basal was in the post-obese 30.5 +/- 1.8% and in the non-obese controls 29.5 +/- 2.3% (NS). In absolute terms, the increments above basal were also similar in the two groups. The heart rate was higher in the post-obese group throughout the study period. It is concluded that the thermogenic response to a mixed meal is much the same in post-obese subjects and in non-obese controls. These findings support the notion that a decreased meal-induced thermogenesis is a secondary phenomenon rather than a primary pathogenic factor in human obesity.