Glucose toxicity is a well-established entity that has been shown in animal models of diabetes to contribute to development of insulin resistance and impaired insulin secretion. In type II (non-insulin-dependent) diabetes in humans, a considerable body of evidence has accumulated indicating that a chronic physiological increment in the plasma glucose concentration leads to progressive impairment in insulin secretion and may contribute to insulin resistance as well. The precise biochemical mechanism(s) responsible for the hyperglycemia-induced defect in insulin secretion remains to be defined but may be related to a defect in phosphoinositide metabolism. In animal models of diabetes, development of insulin resistance is related to downregulation of the glucose-transport system, and a similar phenomenon is also likely to occur in humans. In addition, hyperglycemia in humans may lead to a defect in glycogen synthesis. In this respect, humans may be different from rats. In type I (insulin-dependent) diabetic patients who are poorly controlled, insulin resistance is a characteristic feature and can be ameliorated by tight glycemic control, suggesting that hyperglycemia is responsible for the insulin resistance. Evidence also has accumulated to implicate glucose toxicity in the functional impairment in insulin secretion that occurs during the initial presentation of patients with type I diabetes, and this may explain the honeymoon period so commonly observed after the institution of insulin therapy.