Abstract Plasma immunoreactive insulin levels were measured before and for 6 hr following a 100 g oral glucose load in ten normal volunteers and 17 grossly obese subjects. Eleven of the obese had an abnormal glucose tolerance, five of whom were overt diabetics. Twelve of the obese were restudied after significant weight reduction (thinned obese). Eight thinned obese subjects were also restudied 6–12 mo after completion of the weight reduction protocol. Body composition was measured in each subject prior to testing. Obesity was associated with hyperinsulinemia in the fasting state and in response to oral glucose. The obese diabetics demonstrated a delay and an impairment of insulin secretion in response to glucose. After weight reduction, elevated fasting plasma insulin levels fell in all. Insulin response to oral glucose was not different in the thinned obese with normal glucose tolerance from that observed in the normal volunteers. There was significant correlation between both fasting plasma insulin and total measurable insulin following the glucose load, and total body fat in the obese and thinned obese nondiabetics, but not in the obese overt diabetics. There was, however, significant correlation between fasting plasma insulin levels and total body fat in the diabetics who had a normal fasting blood sugar. These data indicate that the hyperinsulinemia of obesity is clearly related to the increase in total body fat. Carbohydrate intolerance occurs in those obese individuals with a limited pancreatic insulin secretory reserve, which fails to compensate for the increase in total body fat.