We examined the fasting plasma glucose (FPG) levels at which the compensatory mechanism of pancreatic beta cells could act and dyslipidemia would occur in Japanese subjects. A 75 g oral glucose tolerance test (OGTT) was performed on 8923 subjects. The relationship between FPG and immunoreactive insulin (IRI) was analyzed. The mean Sigma IRI (sum of IRI during OGTT) level formed an inverted U-shaped curve with a peak level at FPG of 110-130 mg/dl. The mean IRI level 30 min after glucose administration (as a marker of initial insulin secretion) began to decrease at an FPG level of approximately 100 mg/dl. The mean triglyceride level increased linearly with the elevation of the FPG level at FPG levels of 80-120 mg/dl. The mean high-density lipoprotein-cholesterol level decreased linearly with increasing FPG at FPG levels of 80-110 mg/dl. In Japanese, total insulin secretion during OGTT was affected at FPG levels >110-130 mg/dl. Moreover, subjects with FPG levels >110 mg/dl showed decreased initial insulin secretion and dyslipidemia. These results may indicate the necessity of intervention in individuals with impaired fasting glucose.