OBJECTIVES: To determine the effects of a lifestyle intervention on the development of type 2 diabetes mellitus (T2DM) among participants with impaired glucose tolerance (IGT), in particular in the subgroup with baseline glycated hemoglobin (HbA1c) levels >= 5.7%, in primary healthcare settings. DESIGN: Randomized controlled trial. SETTING: 32 healthcare centers in Japan. PARTICIPANTS: Participants with IGT, aged 30 to 60 years, were randomly assigned to either an intensive lifestyle intervention group (ILG) or a usual care group (UCG). INTERVENTIONS: During the initial 6 months, participants in the ILG received four group sessions on healthy lifestyles by public health providers. An individual session was further conducted biannually during the 3 years. Participants in the UCG received usual care such as one group session on healthy lifestyles. OUTCOME MEASURES: The primary endpoint was the development of T2DM based on an oral glucose tolerance test. RESULTS: The mean follow-up period was 2.3 years. The annual incidence of T2DM were 2.7 and 5.1/100 person-years of follow-up in the ILG (n = 145) and UCG (n = 149), respectively. The cumulative incidence of T2DM was significantly lower in the ILG than in the UCG among participants with HbA1c levels >= 5.7% (log-rank = 3.52, p = 0.06; Breslow = 4.05, p = 0.04; Tarone-Ware = 3.79, p = 0.05), while this was not found among participants with HbA1c levels < 5.7%. CONCLUSIONS: Intensive lifestyle intervention in primary healthcare setting is effective in preventing the development of T2DM in IGT participants with HbA1c levels >= 5.7%, relative to those with HbA1c levels < 5.7%. TRIAL REGISTRATION NUMBER: UMIN000003136.

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