In the past few years the general medical profession has gradually become aware of the existence of a clinical syndrome characterized by the periodic occurrence of spontaneous hypoglycemia with its resulting symptoms relieved dramatically by the administration of sugar. The association of hypoglycemia with an overdose of insulin and the concept of endogenous hyperinsulinism, proposed by Harris1in 1924 and established by Wilder and his co-workers2in 1927 have led to the widespread misconception that hyperinsulinism is synonymous with spontaneous hypoglycemia. To be sure, hyperinsulinism is one of the most important causes of periodic spontaneous hypoglycemia in man. But, as will be indicated, there are many other abnormalities which produce an abnormally low level of the blood sugar and which, consequently, produce the same train of symptoms. Thus, in any case in which it is suspected that symptoms are due to hypoglycemia, the presence of abnormally low levels