When the serum insulin valves during a 50-gm oral glucose tolerance test were expressed as the function of glucose concentrations, a good linear relationship was found in normal subjects and in patients with diabetes, hyperthroidism, hypothroidism or chronic hepatitis. The insulin index calculated from the line representing this relationship was less than 100 in all 19 diabetic patients and most of the 13 hyperthyroid patients with oxyhyperglycemia, whereas it was more than 100 in the 14 normal subjects, the 4 hypothyroid patients, and the 8 patients with oxyhyperglycemia due to chronic hepatitis. The insulin index decreased with increasing maximal blood glucose concentration during the glucose tolerance test in diabetic patients. Furthermore, improvement in the insulin index was correlated with the effectiveness of sulfornylurea drugs in the diabetic group.