Hyperinsulinism may precede diabetes mellitus or occur in the course of that disease, but it also occurs in non-diabetics. The disease is controlled, but not cured, by dieting. An investigation of the fasting blood sugar in 253 cases disclosed that there were 92 with hyperglycemia and 12 with hypoglycemia (below 0.070%). The latter group was amongst non-diabetics and the insulin reaction was excluded. All these latter cases had symptoms of hypoglycemia. In 1867 other cases, 370 were frank diabetics, 51 gave fasting blood sugar readings above 0.120% and these were treated as potential diabetics, while 67 had readings ranging between 0.045 and 0.079% and showed symptoms of hypoglycemia. According to the author, the cause is a permanent lesion in the islets of Langerhans. Published cases on hyperinsulinism and dysinsulinism are reviewed.