Hypoglycemia is commonly defined by a blood sugar level below 50 mg/100 ml. It is a syndrome and can he classified into three types: fasting, reactive, and intermittent. Fasting hypoglycemia is characterized by an abnormally low fasting blood sugar, a poor tolerance of physical exercise, and signs and symptoms attributable predominantly to cerebral dysfunction. The main causes for fasting hypoglycemia are insulin-producing islet cell tumors and extrapancreatic tumors associated with hypoglycemia. Although clinically both are undistinguishable from each other, they can often be separated by laboratory tests and physical examination. Islet cell tumors in a large proportion of cases exhibit elevated serum insulin levels, or at least an increased proinsulin component, in addition to a typical response to tolbutamide or glucagon. Other, but less common, causes of fasting hypoglycemia include liver disease and hypofunction of endocrine glands, such as the pituitary, the thyroid, the adrenal glands, and the alpha cells of the pancreatic islets. The second group of hypoglycemia, which is much more