BACKGROUND Previous studies demonstrated that intensive lifestyle modification can prevent type 2 diabetes mellitus among those with impaired glucose tolerance, but similar beneficial results have not been proved among those with impaired fasting glucose levels. We investigated the efficacy of lifestyle modification on type 2 diabetes incidence among those with impaired fasting glucose levels. METHODS The present study was an unmasked, multicenter, randomized, controlled trial. A total of 641 overweight Japanese (aged 30-60 years) with impaired fasting glucose levels were recruited nationwide in Japan and randomly assigned to a frequent intervention group (n = 311) or a control group (n = 330). For 36 months after randomization, the frequent intervention group received individual instructions and follow-up support for lifestyle modification from the medical staff 9 times. The control group received similar individual instructions 4 times at 12-month intervals during the same period. The primary outcome was type 2 diabetes incidence in annual 75-g oral glucose tolerance tests, diagnosed according to World Health Organization criteria. RESULTS There were no significant differences between the allocation groups in baseline characteristics and dropout rates. Estimated cumulative incidences of type 2 diabetes were 12.2% in the frequent intervention group and 16.6% in the control group. Overall, the adjusted hazard ratio in the frequent intervention group was 0.56 (95% confidence interval, 0.36-0.87). In the post hoc subgroup analyses, the hazard ratio reduced to 0.41 (95% confidence interval, 0.24-0.69) among participants with impaired glucose tolerance at baseline, and to 0.24 (0.12-0.48) among those with baseline hemoglobin A(1c) levels of 5.6% or more (the Japan Diabetes Society method). Such risk reduction was not observed among those with isolated impaired fasting glucose findings or baseline hemoglobin A(1c) levels of less than 5.6%. CONCLUSIONS Lifestyle modifications can prevent type 2 diabetes among overweight Japanese with impaired fasting glucose levels. In addition, identifying individuals with more deteriorated glycemic status by using 75-g oral glucose tolerance test findings or, especially, measurement of hemoglobin A(1c) levels, could enhance the efficacy of lifestyle modifications. BACKGROUND umin.ac.jp/ctr Identifier: UMIN000001959.