Recent reports have shown a lack of agreement between the impaired glucose tolerance (IGT) and the impaired fasting glucose (IFG) categories, suggesting that correspond to different impaired glucose metabolism stages.To determine the differences of serum insulin levels between subjects with IFG and IGT diagnoses.Cross-sectional study of 52 subjects with IFG and 48 with IGT diagnosis, and a euglycemic group of 140 subjects. Serum glucose and insulin were measured in both fasting and 2-h 75-g oral post-load glucose (2-h PG).Subjects with IFG showed the highest fasting and 2-h PG serum insulin levels, whereas subject with IGT the lowest. Serum insulin values showed no significative changes between the fasting and 2-h PG conditions in the subjects with IGT, whereas the subjects with IFG showed significative hyperinsulinemia. The serum glucose 2-h PG showed an increase of 0.2 mmol/l (CI(95%) 0.07-0.33), 0.5 mmol/l (CI(95%) 0.41-0.58) and 3.6 mmol/l (CI(95%) 3.39-3.81) with respect to basal values, whereas the increase of serum insulin 2-h PG was of 54 pmol/l (CI(95%) 53.71-55.29), 918 pmol/l (CI(95%) 917.49-918.51) and 0.5 pmol/l (CI(95%) 0.15-0.84) for the euglycemic, IFG and IGT subjects, respectively.This study demonstrates that subjects with IFG show hyperinsulinemia whereas those with IGT have low insulin secretion in response to oral load glucose, suggesting that IFG and IGT correspond to different stages of impaired glucose metabolism.