We evaluated the predictors of the development from normal to impaired glucose tolerance (IGT) in healthy middle-aged Japanese men. Forty male subjects who showed normal glucose tolerance (NGT) levels based on WHO criteria and who had undergone 75-g OGTT annually for 10 years were selected in the database of medical checkups retrospectively, and divided into two groups: those retaining NGT and those that developed IGT. Gamma-glutamyl transpeptidase (GGT) and the glucose levels at 30 and 60 min were significantly associated with the development of IGT in the Cox proportional hazard model. However, other clinical characteristics and the glucose levels at pre-load and at 120 min were not significantly associated with the development of IGT. GGT and the glucose levels at 30 and 60 min after the 75-g glucose load were predictors of development from NGT to IGT in healthy middle-aged Japanese men.