Knowing the relationship between obesity and diabetes, the purpose of our work was to study the alterations in lipid metabolism as measured by continuous indirect calorimetry in the course of a 100-g oral glucose-tolerance test in groups of obese patients without and with diabetes, respectively. Seventy-nine obese patients participated in the study. They were divided into four groups according to the degree of carbohydrate intolerance: group A, normal glucose tolerance; group B, impaired glucose tolerance; group C, diabetes with hyperinsulinemic response to the load; group D, diabetes with impaired insulin response. All four groups of patients presented an increase in lipid oxidation, both in the fasting state and during the three-hour glucose tolerance test, when compared to the control group. The lipid oxidation rate was roughly parallel to plasma free fatty acid (FFA) levels. The contribution of lipids to energy expenditure was higher in obese as compared to control subjects. These observations suggest that the larger part taken by lipids in the energy metabolism of both nondiabetic and diabetic obese humans is a consequence of their increased fat stores and that the resulting decrease in carbohydrate metabolism may lead, as a late consequence, to alterations in glucose tolerance. The latter may result in delayed glucose storage and oxidation in the obese patient.