The insulin response during OGTT was studied in 721 subjects with normal or mildly impaired glucose tolerance. Since the serum IRI level depends on the serum glucose level, and is widely distributed at a given serum glucose level, it is not very meaningful to compare the serum IRI level by itself. In the present study, the following 5 indices were utilized for evaluating the insulin response: a) the insulinogenic index (II), i.e. the ratio of the increment of IRI over that of glucose during 30 min after glucose loading in OGTT, b) the glucose-insulin coefficient (GIC), i.e. the regression coefficient of IRI on glucose during OGTT as estimated on an individual basis, c) the ratio of IRI area over glucose area during OGTT (RIG), d) the ratio of IRI area during 30 min after glucose loading over that during the entire OGTT period (RI₃₀), and e) the type of IRI curve as classified according to the time of the peak value.<BR>II and RI₃₀ are indices related to acute insulin response, and are sufficiently sensitive to evaluate the insulin response in subjects with mild glucose intolerance. On the other hand, RIG is an index related to the total amount of insulin released during OGTT, and is appropriate for evaluating the insulin response in subjects with considerably advanced glucose intolerance. GIC has an intermediate character between the above two kinds of indices. It behaved almost identically to II or RI₃₀. Such results were also confirmed by principal component analysis.<BR>According to the level of the IRI area (greater or less than 100) and the level of II (greater or less than 0.6), the subjects were classified into 4 patterns. These were: A, diminished response; B, normal response; C, delayed response; and D, excessive response. In subjects with borderline glucose intolerance, patterns C and D were predominant. In those with more advanced glucose intolerance, patterns A and C were predominant. However, it was of interest to note that with regard to their future prognosis, some subjects indicating pattern A or C were present among those with normal glucose tolerance.<BR>In conclusion, it can be said that the insulin response during OGTT indicated a variety of IRI patterns even within the same glucose tolerance group, the meanings of which remained obscure. Further studies in this regard should thus be made.