Preoperative localization of an insulinoma was possible using a technique of transhepatic pancreatic venography, with measurement of insulin levels in six pancreatic veins. Surgical exploration confirmed the presence of an insulinoma at the site predicted by venography. Peripheral plasma insulin levels were only minimally elevated at the time of hypoglycemia. However, complete suppression of ketogenesis during a 74-hour fast was observed, and felt to be a biological marker for hyperinsulinism. These observations suggest that transhepatic pancreatic venography and the measurement of plasma ketones in addition to glucose and insulin during fasting, may be useful supplementary studies in the evaluation of patients with fasting hypoglycemia.