Background:Atrial fibrillation (AF) occurs more frequently among patients with renal dysfunction. We investigated the possible association between prevalence of AF and serum fibroblast growth factor 23 (FGF23), which has been shown to be increased in subjects with renal dysfunction.Methods and Results:Among the total enrollment of 851 cardiac patients, 188 patients had AF (paroxysmal AF, 95; non-paroxysmal AF, 93). Prevalence of AF for FGF23 octile had a U-shaped relationship with the lowest prevalence at the fifth octile. On logistic regression analysis, when the third FGF23 quartile was used as the reference, the first and fourth FGF23 quartiles were associated with prevalence of AF with an odds ratio (OR) of 3.34 (95% confidence interval [CI]: 1.89–5.88) and 2.58 (95% CI: 1.45–4.58), respectively, after adjusting for confounding factors including estimated glomerular filtration rate (eGFR). Among the subgroup of 416 patients for whom serum parathyroid hormone and 25-hydroxy vitamin D data were available, OR of the first and the fourth FGF23 quartile were calculated to be 3.52 and 2.97, respectively, when further adjusted for these two variables in the statistical model.Conclusions:Serum FGF23 had a U-shaped relationship with prevalence of AF among Japanese cardiac patients, which was independent of other calcium/phosphate metabolism-related parameters and eGFR. Pathophysiology underlying the observed link, if at all, awaits further investigation. (Circ J 2015; 79: 1742–1748)