This report deals with a Japanese male patient who showed severe spontaneous hypoglycemic attacks and mild diabetes. When he was 47-year-old, the first attack of unconsciousness occurred, and after that he was suffering from the same attacks for 5 years. After 3 years from the first attack mild diabetes was found. He was admitted in June, 1968, when he was 52-year-old. Although the patient had never been treated with insulin, his serum showed insulin-binding capacity over 3 years under our observation. A large amount of insulin determined as immunoreactive insulin was extracted with acid ethanol from his native serum. A binding reaction between the insulin-free gamma globulin fraction of the serum and pig ¹³¹I-insulin was more inhibited by crystalline human insulin than by crystalline beef insulin. No insuloma was found, but one-third of the pancreas was excised on August 21, 1968. In the histological examination a hyperplasia of the pancreatic islets was significant and there was no inflammatory reactions in the islets. After the operation the hypoglycemic attacks ceased, but the insulin-binding capacity of the serum and mild diabetic state were still observed in July, 1971.