To determine the contribution of obesity to the insulin resistance of non-insulin-dependent diabetes mellitus, insulin dose response curves for suppression of glucose production and stimulation of glucose utilization were generated in lean and obese diabetic patients and compared to those observed in weight-matched nondiabetic subjects. Glucose utilization during 0.4, 1.0, and 10.0 mU/kg x min insulin infusions (producing insulin concentrations ranging from approximately 50 to 2,000 microU/mL) was lower (p less than .02 to .001) in lean and obese diabetic patients compared to weight-matched nondiabetic subjects indicating insulin resistance. Glucose utilization was not correlated with obesity in the diabetic subjects. Suppression of glucose production was impaired (P less than .03 and .001) in both the lean and obese diabetic subjects at physiologic but not supraphysiologic insulin concentrations. We conclude that patients with NIDDM have both hepatic and extrahepatic insulin resistance, the severity of which appears to be independent of the degree of obesity.