In 2003, the American diabetes Association (ADA) recommended that the definition of impaired fasting glucose (IFG) be changed from 110-125 to 100-125 mg/dL. This study examined the effect of different cut points for IFG on the incidence of diabetes in Tehran’s urban population.Materials and Methods: From among the participants of the Tehran Lipid Glucose Study, after excluding subjects aged<20 years, those with known or newly diagnosed type 2 diabetes, and those with missing values of weight, height, or other variables or lost to follow-up, data of 4728 subjects was used in this cohort study. They were followed for diabetes incidence (based on fasting plasma glucose (FPG) and glucose tolerance test) for 3.6 years. Participants were divided into different groups: Normoglycemia<100, original-IFG 110-125, added IFG 100-110 and the new IFG 100-125 mg/dL, groups. Odds ratios of diabetes incidence after adjustment for relevant confounders were calculated.Results: The median age of participants was 42.9±13.7y and 59.1% (n=2916) were female. The prevalence of original-IFG, added IFG and new IFG were 3.7% (n=183), 11.8 % (n=584), and 15.5% (n=767), respectively.After a mean follow-up duration of 3.6 years, 188 cases (3.8%) of incident diabetes were diagnosed. Diabetes incidence in the normoglycemia, original-IFG, added IFG and new IFG groups, were 1.8% (n=76), 26.2% (n=48), 11% (n=64), 14.6 % (n=112), respectively. Odds ratio for diabetes incidence after adjustment for age, sex and other relevant confounders for the original-IFG, added IFG and new IFG groups, compared to the normoglycemia group as the reference, were 11.45 [95% confidence interval (CI), 7.45-17.57], 4.73 (95% CI, 3.28-6.8), 6.32 (95% CI, 4.51-8.5), respectively.Conclusion: The new IFG (100-125 mg/dL) is not superior to the original IFG (110-125 mg/dL) in terms of predicting forthcoming diabetes in Tehranian adults.