Previous studies have shown that patients with non-insulin dependent diabetes mellitus (NIDDM) have a higher metabolic rate (RMR) and lower thermogenesis in comparison with persons with normal glucose tolerance. It is not clear whether this impairment is due to the diabetic state per se or to the association of the diabetic and obese state. The impact of obesity on RMR and glucose-induced thermogenesis (GIT) was studied in seven non-obese and 12 obese men with NIDDM; the results are compared with a group of six obese men with normal glucose tolerance. RMR was significantly higher for the obese subjects (P < 0.02) but this difference disappeared after correction for fat-free mass. Mean GIT was significantly lower (P < 0.01) in the diabetic patients, whether they were obese or non-obese. The results of this study indicate that for patients with NIDDM, the impact of obesity on both RMR and GIT is rather limited. On the other hand, a significant influence of glucose tolerance on GIT in obese patients could be demonstrated.