The glucose-induced thermogenesis (GIT) following a 100-g oral glucose load has been measured by continuous indirect calorimetry in 55 nondiabetic and diabetic obese subjects of various ages and compared with two control groups of 17 young and 13 elderly nonobese subjects. The obese subjects were divided into four groups: group A, normal glucose tolerance; group B, impaired glucose tolerance; group C, diabetes with increased insulin response; group D, diabetes with reduced insulin response. The glucose-induced thermogenesis measured during 3 h represented 8.6 ± 0.7% of the energy content of the load in the young control group. In all obese groups, the glucose-induced thermogenesis was significantly lower than in the young control group, i.e., 6.6 ± 0.9%, 6.4 ± 0.6%, 3.7 ± 0.7%, and 2.2 ± 0.4% in groups A, B, C, and D, respectively. Since the obese diabetics were older than the other groups, their glucose-induced thermogenesis was compared with that of the elderly control group; the latter (5.8 ± 0.3%) was significantly lower (P < 0.05) than that of the young control group. The obese diabetics also had a significantly lower glucose-induced thermogenesis than the elderly control group (P < 0.02 and P < 0.001 for groups C and D, respectively). When corrected for glucosuria and after taking into account the glucosuria and the changes in the glucose space, the corrected glucose-induced thermogenesis (i.e., related to glucose “uptake”), was still significantly reduced in group A versus the young control group (6.6 ± 0.9 versus 8.6 ± 0.7%, P < 0.05), and in group D versus the elderly (matched for age) control group (4.2 ± 0.7 versus 5.8 ± 0.3%, P < 0.05). It is concluded that the postprandial thermogenesis induced by glucose ingestion is decreased in the presence of insulin resistance and/or reduced insulin response to the glucose load in obese subjects. In addition, age itself contributes to decrease glucose-induced thermogenesis.