We have examined the contribution of insulin secretion and insulin resistance to glucose intolerance in Japanese. Some indices of insulin secretion and insulin sensitivity based on the results of OGTT were used. The decline of insulin secretion capacity was significant throughout the development of glucose intolerance from NGT via IGT to DM. Decreased insulinogenic indices were conspicuous when it is compared with other types of diabetes. Slight impairment of insulin secretion has begun in subjects with NGT. The progression from NGT via isolated IGT to isolated post-challenge hyperglycemia was considered mostly due to the deterioration of early-phase insulin secretion. It is summarized that decreased insulin secretion capacity takes a definite role in the development from NGT to type 2 diabetes in Japan.