Early phase insulin secretion has been reported to be impaired in patients with type 2 diabetes and to remain subnormal even after treatment of the glucotoxicity. We evaluated insulin secretion profiles by performing a 75-g oral glucose tolerance test (OGTT) in 178 Japanese patients with type 2 diabetes after minimizing the influence of glucotoxicity with diet or insulin therapy during hospitalization. Among the 178 patients, 161 (90.4%) had a low insulinogenic index (I.I.; < 0.4) and 17 (9.6%) had an index in the normal range (I.I. ≥ 0.4). This normal I.I. group was characterized by visceral obesity and insulin resistance, and showed delayed insulin secretion. Their total insulin response was significantly increased compared with that of the low response group or the normal glucose tolerance (NGT) group. Strikingly, in the normal I.I. group, one patient improved to the stage of NGT and three to impaired glucose tolerance (IGT) after treatment of the glucotoxicity. These results indicate that some patients will show a normal insulinogenic index after treatment of the glucotoxicity.