Objectives: Borderline diabetes, a precursory condition of diabetes, is an important issue in the prevention of diabetes. The aim of the present study was to clarify the effects of one-year changes in obesity and exercise on the development of diabetes or return to normal fasting plasma glucose (FPG) levels among middle-aged people with impaired fasting glucose (IFG) at baseline. Methods: Among those who attended a basic health examination in 1997, we selected 1,620 subjects who showed impaired fasting glucose (FPG of 110 mg/dl or higher) and had complete data on height, weight and exercise. At the one-year follow-up (in 1998), 1,099 of those subjects attended a health examination; FPC, height, weight and exercise were evaluated for 731 subjects. Subjects were classified into the following three groups, on the basis of changes in FPG during the year between the two examinations: developing diabetes (DM), remaining IFG, and returning to normal (WNL). Results: Among those who were initially obese, there was a significant difference in the proportions of DM, IFG and WNL between those with and without improvement in obesity in the year preceding the follow-up (p<0.05). Those with improvement in obesity showed a significantly higher tendency to return to WNL than those without improvement in obesity. Multiple logistic regression analysis showed that those with improvement in obesity had a significantly higher odds ratio (2.17) to return to WNL (p=0.015). Among those who were initially not obese, there was no significant association between changes in obesity and developing DM or returning to WNL. No significant association was observed between changes in exercise and developing DM or returning to WNL. Conclusion: The present findings suggest that, among obese IFG subjects, improvement in obesity is associated with returning to normal plasma glucose. Weight control may be important for the normalization of borderline diabetes.