To investigate the effect of postprandial plasma glucose (PG) concentrations on HbA1c levels in type 2 diabetic patients, we evaluated the relationship between HbA1c levels and postprandial PG concentrations after a meal tolerance test in 35 type 2 diabetic patients who had fasting PG concentrations persistently < 7.8 mmol/l and stable HbA1c levels. Two-hour postprandial PG concentrations were found to be more strongly correlated (r = 0.51) with HbA1c levels than 1-h postprandial PG (r = 0.35) and fasting PG (r = 0.46) concentrations. Patients whose HbA1c levels were high (HbA1c > or = 7%) had significantly higher 2-h postprandial PG concentrations and areas under the glucose curve than those whose HbA1c levels were lower (8.12+/-1.10 (SD) vs 6.70+/-2.22 mmol l(-1), P = 0.004 and 17.43+/-1.92 vs 15.58+/-3.26 mmol h(-1) l(-1), P = 0.02, respectively). Although fasting PG concentrations of patients with higher HbA1c levels were slightly higher, they did not differ significantly from those with lower HbA1c levels (6.21+/-0.89 vs 5.73+/-0.68 mmol l(-1)). Age, duration of diabetes, body mass index, serum C-peptide, both fasting and postprandial, did not differ between these two groups. This study suggests that postprandial hyperglycemia, particularly 2-h postprandial PG concentrations, is associated with high HbA1c levels in type 2 diabetic patients whose fasting PG levels were within normal or near-normal levels.