The discriminating abilities of fasting plasma glucose (FPG) and HbA1c were compared on screening tests for impaired glucose tolerance (IGT) and IGT plus diabetes mellitus by the receiver operating characteristic (ROC) curve analysis. Furthermore, effects of sex, age and BMI were examined on sensitivity and specificity of the optimal cutoff points. This study included 997 subjects who were recruited for 75 g OGTT after the first screening of the Japan Diabetes Prevention Program. According to the 1997 criteria of the American Diabetes Association (ADA), 140 subjects were classified as diabetic and 256 as IGT. The areas under the ROC curves of FPG were significantly larger than those of HbA1c. The optimal cutoff points of FPG were 102 mg/dl for IGT and 105 mg/dl for IGT plus diabetes mellitus. Those of HbA1c were both 5.3%. In screening with FPG, females had significantly lower sensitivity and higher specificity than males, and the specificity for IGT plus diabetes mellitus was the lowest in the obese group. In screening with HbA1c, the specificity was low in the older and the obese groups. We concluded that FPG was superior to HbA1c for screening of IGT and IGT plus diabetes mellitus and the optimal cutoff point of FPG would be 102 mg/dl or greater.