The aim of the study was to assess the screening test properties of HbA1c for undiagnosed diabetes (DM) according to the 1999-WHO criteria and its relevance of the Japan National Diabetes Survey Cut-off points for possible and probable DM: HbA1c >or=5.6 and 6.1%. Screening properties of HbA1c predicting undiagnosed DM was examined and compared with that of fasting plasma glucose (FPG) in 1904 Funagata-town inhabitants aged 35-89 years old. The prevalence of previous DM, undiagnosed DM, and impaired glucose regulation (IGR) were 5.5, 6.0, and 18.6%, while the prevalence of probable and possible DM were 7.7 and 5.4%. The area under the receiver operating characteristic curve for undiagnosed DM was similar between HbA1c (0.856 [95% CI: 0.812-0.899]) and FPG (0.902 [0.869-0.936]). HbA1c of 5.6% gave a sensitivity of 56.5%, a specificity of 95.1%, positive and negative predictive values of 44.2 and 97.0%, and a proportion of people above the cut-off point of 8.2%. True positive tests were significantly higher with mean levels of BMI, fasting, and 2-h plasma glucose, and HbA1c, but lower with mean levels of high-density-lipoprotein cholesterol than in false negative tests. The measurement of HbA1c alone may be efficient to screen undiagnosed DM and the cut-off point of 5.6% might be proper with respect to screening tests properties for undiagnosed DM, and prediction of vascular complications in Japan.