BACKGROUND: Subjects with normal glucose tolerance (NGT) who have a high 1-hour postload plasma glucose level (> or =155 mg/dL; NGT 1 hour-high) have been shown to be at higher risk for type 2 diabetes than subjects with NGT 1 hour-low postload plasma glucose level (<155 mg/dL). We compared beta-cell function in subjects with NGT 1 hour-high, NGT 1 hour-low, and impaired glucose tolerance (IGT). METHODS: We classified subjects into NGT 1 hour-low (n=149), NGT 1 hour-high (n=43), and IGT (n=52). The beta-cell function was assessed based on insulinogenic index (IGI), oral disposition index (DI), and insulin secretion-sensitivity index-2 (ISSI-2). RESULTS: Insulin sensitivity was comparable between the subjects with NGT 1 hour-high and NGT 1 hour-low. The beta-cell function with/without adjusting insulin sensitivity was significantly different among the three groups. The IGI (pmol/mmol) was 116.8+/-107.3 vs. 64.8+/-47.8 vs. 65.8+/-80.6 (P=0.141), oral DI was 3.5+/-4.2 vs. 1.8+/-1.4 vs. 1.8+/-3.1 (P<0.001), and ISSI-2 was 301.2+/-113.7 vs. 213.2+/-67.3 vs. 172.5+/-87.5 (P<0.001) in NGT 1 hour-low, NGT 1 hour-high, and IGT, respectively. Post hoc analyses revealed that oral DI and ISSI-2 were significantly different between NGT 1 hour-low and NGT 1 hour-high but comparable between NGT 1 hour-high and IGT. CONCLUSION: Among Korean subjects with NGT, those who have a higher 1-hour postload glucose level have a compromised insulin-sensitivity adjusted beta-cell function to a similar degree as IGT subjects.