In three patients who had repeated episodes of symptomatic postprandial hypoglycemia, oral glucose tolerance testing demonstrated normal initial insulin response but subsequent hyperinsulinemia and hypoglycemia. Intravenous glucose and tolbutamide provoked normal glucose and insulin responses, suggesting normal beta-cell function. Evidence that the hyperinsulinism and hypoglycemia provoked by oral glucose was due to alimentary factors included hyperglycemia, hyperinsulinism, and subsequent hypoglycemia after duodenal glucose infusions, normal gastric emptying as measured by a radioisotopic method, and control of the hyperglycemic, hyperinsulinemic responses and of hypoglycemic symptoms in these patients by phenformin, a drug reported to impede glucose absorption from the gut. We conclude that alimentary hypoglycemia may occur in the absence of gastrointestinal surgery, and can be distinguished from the hypoglycemia of early diabetes by normal to increased initial insulin secretion after the ingestion of glucose. (N Engl J Med 288:1206–1210, 1973).