The incidence and risk factors of nonalcoholic fatty liver disease (NAFLD) have never been prospectively determined. To determine the frequency and risk factors of NAFLD and chronological ordering between NAFLD, weight gain, and features of insulin resistance, a historical cohort study was conducted in a Japanese workplace. A cohort free of previous liver injury, alcohol consumption of more than 140 g/wk, and hepatitis B or C infection (529 of 1537 subjects), and a subcohort of 287 subjects free of insulin resistance-related features were identified. Elevated aminotransferases in nonalcoholics were used as a surrogate for NAFLD. High aminotransferases together with weight gain of more than 2 kg and insulin resistance-related features in the subcohort were sought for up to 5 years. The incidence of high aminotransferases was 31 per 1000 person-years (71 events). A significant interaction occurred between age and sex in the development of high aminotransferases. In subjects younger than age 40 years, male sex (hazard ratio [HR]: 4.6), elevated body mass index (HR: 2.1), hypertension (HR: 2.6), and low high-density lipoprotein cholesterol (HR: 2.8) increased the risk of high aminotransferases, whereas age (HR: 0.6 for each 5 years) decreased the risk. In subjects older than age 40 years, glucose intolerance (HR: 5.3) was the only significant risk factor. In the subcohort, weight gain preceded high aminotransferases and other insulin resistance-related features, which appeared sequentially in order of low high-density lipoprotein cholesterol, hypertriglyceridemia/hypertransaminasemia/hypertension, and glucose intolerance. In conclusion, this cohort study clearly showed chronological ordering and an association between development of elevated aminotransferases and risk factors of NAFLD.