The population attributable fraction (PAF) of risk factors for the worsening of glucose metabolism in subjects with normal glucose tolerance (NGT) has not been calculated. Our aim was to obtain the PAF of increased body mass index (BMI) and attenuated beta-cell function (BCF) on worsening of glucose metabolism in subjects with NGT. We longitudinally analyzed 604 Japanese adults. The follow-up glucose tolerance status was determined 3.7 years later: 430 participants remained in the NGT category and 102 had progressed to impaired fasting glucose, 67 to impaired glucose tolerance, and 5 to diabetes mellitus. A product of ISIMatsuda and Stumvoll-1, i.e., oral disposition index (DIO), was used as a measure of BCF. The optimal cutoff baseline BMI and DIO values for the prediction of the worsening of glucose metabolism were > 23.1 and < 7.299 kg/m(2), respectively. Isolated increased BMI (iBMIHIGH), isolated low DI (iDIOLOW), and "BMIHIGH and DIOLOW (BMIHIGH/DIOLOW)" were all independently related to the worsening, and the PAF values (95 % CI) for worsening due to iBMIHIGH, iDIOLOW, and BMIHIGH/DIOLOW were 12.9 (3.2-18.4) %, 10.9 (5.0-13.9) %, and 31.4 (22.7-36.3) %, respectively. As much as 55 % of the worsening of glucose metabolism in the NGT subjects was attributable to increased BMI and/or attenuated BCF. The optimal cutoff for BMI was as low as 23.1 kg/m(2) in this population. We believe that these data should form the basis of future public health strategies for the prevention of diabetes in Japan.