Plasma insulin response to glucose infusion was measured in obese subjects with normal and decreased intravenous glucose tolerance. In obese non-diabetic subjects there was insulin hyperresponsiveness to glucose accompanied by peripheral resistance to endogenous insulin. In the obese diabetic subjects insulin response was of the type seen in non-obese diabetics; in no such instance could insulin hyperresponsiveness to glucose be obtained. It is suggested that obesity precipitates diabetes only in subjects with preexisting impairment of insulin response, i. e. in prediabetics. Subjects with unimpaired insulin secreting capacity would overcome the diabetogenic effect of obesity by compensatory hyperinsulinism.