AbstractOrganic hypoglycemia may be caused by an insulinoma. This is a rare tumor, multiple in about 10% and malignant in about 10% of cases. Multiplicity is particularly likely when insulinoma occurs as part of the MEN 1 syndrome. When a patient exhibits Whipple's triad, synchronous blood sugar and insulin estimations must be made, and repeated during fasting. Diagnosis depends on the finding of inappropriately high insulin levels in the presence of low blood sugars. Many stimulatory tests are available but none are as reliable as prolonged starvation, and probably should be abandoned.Measurement of proinsulin and C‐peptide are of particular value in the diagnosis and in elimination of factitious hypoglycemia.Preoperative localization is most useful and can be achieved in up to 90% of patients by skillful selective angiography. Percutaneous portal venous sampling has been used and is particularly indicated in patients after a fruitless first exploration.Tumors of the body and tail are usually treated by distal pancreatectomy, those of the head by enucleation. There is a risk of pancreatic fistula, and duct damage can be unmasked by secretin injection. Occult tumors and hyperplasia may require “blind” progressive distal resection with blood sugar monitoring until the hypoglycemia is controlled, but this is something of a desperation measure. In malignant tumors, hypoglycemia may be controlled by diazoxide or somatostatin. Streptozotocin may control the syndrome and lead to prolongation of life. Like other malignant apudomas, these tumors are usually slow growing and relatively benign.