To evaluate whether beta-cell hyperfunction characterizes glucose intolerant states per se independent of fasting glycemia, we conducted a case-control study among 430 subjects who were classified, by NDGG criteria, as having normal glucose tolerance (n = 230, 130M/130F), nondiagnostic tolerance (NDT, n = 100, 50M/50F) and impaired glucose tolerance (IGT, n = 100, 50M/50F). Thirty-four subjects (17M/17F) with normal glucose tolerance were matched by age, sex, body mass index (BMI), waist-to-hip ratio (WHR), fasting glucose and HbA1c with 30 NDT (15M/15F) and 30 IGT (15M/15F) subjects. The continuous and significant increase in insulin and C-peptide levels across categories of glucose tolerance (from normal to NDT to IGT) was no longer evident in the case-control study: at a fasting plasma glucose ranging from 5.2-5.5 mmol/L (HbA1c was 5%) the concentration of fasting C-peptide was 0.793 +/- 225 nmol/L (mean +/- SD) in subjects with normal glucose tolerance, 0.805 +/- 200 nmol/L in NDT and 0.807 +/- 231 nmol/L in IGT subjects (p = NS). Similarly, plasma concentrations of triglycerides and blood pressure values were similar when subjects of different categories were compared at the same level of glycemia. Sixteen normal subjects were rendered mildly hyperglycemic by a 24-h glucose infusion to match the fasting glucose level of NDT (1 mg/kg/min) and IGT (2 mg/kg/min) subjects. At the same fasting glucose level, normal subjects presented elevations of fasting C-peptide significantly (p < 0.01) higher than subjects belonging to the NDT and IGT categories.(ABSTRACT TRUNCATED AT 250 WORDS)