Sixty-seven subjects with moderate obesity (50 +/- 3 percent above ideal body weight) were given an oral glucose tolerance test with the simultaneous measurement of rates of glucose and lipid oxidation by continuous indirect calorimetry. When the subjects were stratified into nine 5-year classes of duration of obesity, the prevalence of impaired glucose tolerance (IGT) and overt diabetes both increased with increasing duration of obesity. Both basal and post-OGTT lipid oxidation rates were, however, similar in all classes. To assess the independent influence of IGT, diabetes, age, and duration of obesity on glucose metabolism, the data were subjected to analysis of variance using a factorial design with metric covariates. Age by itself was found to be associated (P less than 0.05) with a decline in total post-OGTT glucose oxidation. Both IGT and diabetes, on the other hand, were associated with increased plasma insulin and free fatty acid (FFA) levels, both in the fasting state and following glucose ingestion (P = 0.05-P less than 0.002). Only diabetes, however, was associated with a drastic reduction in nonoxidative glucose disposal, which marked the appearance of, and strongly correlated with (r = -0.81, P less than 0.001), fasting hyperglycemia. Duration of obesity had significant metabolic consequences in its own right: a fall in the insulin response to glucose (P = 0.05) and in the rate of total glucose oxidation (P = 0.03), and a rise in post-OGTT glucose levels (P = 0.04). We conclude that: (a) increased lipid oxidation is common in obesity, but is not sufficient to explain the deterioration of glucose tolerance in long-term obesity; (b) very-long-term obesity may be associated with partial exhaustion of the beta cell, and the resultant insulinopenia may cause depressed glucose oxidation and impaired glucose tolerance, and (c) a defect in nonoxidative glucose disposal is a characteristic feature of frank diabetes at any stage of obesity.