Elevated serum ferritin levels are associated with insulin resistance, type 2 diabetes, cardiovascular disease, and metabolic syndrome. To date, however, no cohort studies have examined whether serum ferritin levels are an independent risk factor for the obesity. Therefore, we conducted a prospective cohort study to evaluate the temporal relationship between serum ferritin levels and obesity development in Korean men. Total 17,812 healthy Korean men who participated in a medical health check-up program in 2005 were followed-up until 2010. Obesity was defined as a body mass index ≥25 kg/m(2). Cox proportional hazards model was used to measure the hazard ratio of the quartile groups of serum ferritin levels. During 64,446.5 person-years of follow-up carried out, 2,627 patients became obese. After adjusting for multiple covariates, we found that the hazard ratios (95% confidence interval) for incident obesity when we compared the second, third and fourth quartiles of serum ferritin levels with the first quartile were 1.08 (0.95-1.23), 1.14 (1.00-1.30), and 1.24 (1.09-1.41), respectively (p for trend = 0.003). Both severe obesity (body mass index ≥ 30 kg/m(2)) and abdominal obesity based on waist circumference (>90 cm) showed consistent longitudinal associations (p for trend <0.001). Elevated serum ferritin levels may have been a predictive factor for obesity during the 5-year follow-up in 17,812 Korean men.