Although the adverse cardiovascular effect of anemia has been well described, the effect of polycythemia on the incident atrial fibrillation (AF) remain unclear. The objective of this study is to identify the association between increased hemoglobin and incident AF. This was a retrospective-cohort study with 434,269 subjects who underwent national health examinations from the Korean National Sample Cohort. We estimated the risk of incident AF according to hemoglobin-based four-categories. During 3.9-year of follow-up, polycythemia group showed higher incidences of AF (hazard ratio[HR] with 95% confidence interval[CI], 1.50 [1.28-1.76] and 1.69 [1.13-2.56]; in men and women, respectively) than normal hemoglobin group (each p<0.001). In the normal hemoglobin and polycythemia groups, a 1 g/dL increase in hemoglobin level was associated with increased risks of incident AF (1.12 [1.07-1.17] and 1.18 [1.10-1.26] in men and women, each p<0.001). To investigate the specific hemoglobin concentration related to greater AF incidence, we analyzed the sensitivity/specificity of different hemoglobin levels: ≥16.0 g/dL in men and ≥14.5 g/dL in women showed the highest Youden's index, with c-indices of 0.83 and 0.82, respectively. Kaplan-Meier cumulative-event curves according to these specific hemoglobin levels (≥16.0 g/dL in men and ≥14.5 g/dL in women) also showed consistent results in both sexes (each p<0.05). Even in the Korean general population, increased hemoglobin was significantly associated with higher rate of incident AF. Especially, subjects with hemoglobin levels ≥14.5 g/dL in women and ≥16.0 g/dL among men were associated with increased risk of incident AF. Copyright © 2020. The Korean Society of Cardiology.