BACKGROUND: Moderately increased plasma ferritin, as a biomarker of iron overload, has been associated with higher rates of cardiovascular death and heart failure. However, the association of moderately increased plasma ferritin with risk of atrial fibrillation in the general population is unknown. METHODS: We examined the association of plasma ferritin concentrations with risk of atrial fibrillation and heart failure in metaanalyses of 35 799 men and women from 3 studies of the Danish general population: the Copenhagen City Heart Study, the Danish General Suburban Population Study, and the Copenhagen General Population Study. RESULTS: Multivariable adjusted fixed effects odds ratios for atrial fibrillation were 1.23 (95% CI, 1.05–1.44; P 0.005) in men for ferritin concentration 300 g/L vs 300 g/L, 1.13 (95% CI, 0.93–1.38; P 0.22) in women for 200 g/L vs 200 g/L, and 1.19 (95% CI, 1.06 –1.35; P 0.005) in both sexes combined (P _{sex interaction} 0.52). Corresponding fixed effects odds ratios for heart failure were 1.16 (95% CI, 0.98 –1.37; P 0.08) in men, 0.86 (95% CI, 0.67–1.10; P 0.23) in women, and 1.05 (95% CI, 0.91–1.21; P 0.45) in both sexes combined (P _{sex interaction} 0.05). Multivariable adjusted fixed effects odds ratio for atrial fibrillation per step increase in ferritin concentrations was 1.13 (95% CI, 1.06 –1.21; P _trend 0.0005) in both sexes combined (P _{sex interaction} 0.59); the corresponding value for heart failure was 1.03 (95% CI, 0.95–1.11; P _trend 0.47) (P _{sex interaction} 0.08). In sensitivity analyses, there was no evidence of U-shaped relationships between plasma ferritin concentrations and risk of atrial fibrillation or heart failure in men or women. CONCLUSIONS: Increased ferritin concentration is associated with increased risk of atrial fibrillation in the general population.