Objective: In Caucasians, plasma glucose concentration at 1 h during an oral glucose tolerance test (OGTT) may be a better predictor of future diabetes mellitus than the fasting or 2-h postload glucose concentration. We investigated whether the 1-h glucose concentration could be used to predict future diabetes mellitus in Asian ethnicity. Measurements: A total of 5703 Koreans with normal glucose tolerance were enrolled from the Korean Genome and Epidemiology Study. Indices of insulin sensitivity and β-cell function estimated from standard 75-g OGTTs performed every 2 years for 12 years were used to identify whether the 1-h glucose concentration could predict future diabetes mellitus. Results: The mean age and body mass index at baseline were 51·3 ± 8·7 years and 24·2 ± 3·0 kg/m2, respectively. During the 12-year follow-up, 593 subjects (10·3%) developed diabetes mellitus. The area under the receiver-operating characteristic curve for incident diabetes mellitus was higher for the 1-h postload glucose concentration than for the fasting or postload 2-h glucose concentration (0·74 vs 0·61 or 0·63). The cut-off value of ≥8·0 mmol/l identified incident diabetes mellitus with 70% sensitivity and 68% specificity. After adjusting for typical risk factors, subjects with a 1-h postload glucose concentration ≥8·0 mmol/l had lower β-cell function and a 2·84-fold increased risk of incident diabetes mellitus compared with their counterparts. Conclusions: In this community-based 12-year prospective cohort study, 1-h postload plasma glucose concentration was an independent predictor of future diabetes mellitus and 8·0 mmol/l was suggested as a cut-off value.