['To determine the association between nonalcoholic fatty liver disease and the risk for development of diabetes.', 'We conducted an observational cohort study in male workers > or = 40 years old in a Japanese company from 1997 to 2005. We excluded workers with alcohol intake > or = 20 g/day and those with impaired glucose tolerance by a 75-g oral glucose tolerance test. The remaining 3,189 workers were classified into fatty liver (FL) and non-FL group based on the findings of abdominal ultrasonography. Both groups were followed for the development of diabetes. Hazard ratio (HR) was determined in Cox proportional hazard analysis. A nested case-control study was conducted to determine the odds ratio (OR).', 'The average age of participants was 48.0 years at the entry, and the average follow-up period was 4.0 years. The incidence of diabetes in the FL group was 2,073 per 100,000 person-years (65 cases), whereas 452 per 100,000 person-years (44 cases) in the non-FL group. The age- and BMI-adjusted HR of diabetes associated with FL was 5.5 (95% CI 3.6-8.5, P < 0.001). In the nested case-control analysis, the OR adjusted for age and BMI was 4.6 (3.0-6.9, P < 0.001).'] Is nonalcoholic fatty liver disease a risk factor for type 2 diabetes in middle-aged Japanese men?