Background: Fibroblast growth factor 23 (FGF-23) is a novel bone-derived phosphate-regulating hormone, and serum FGF-23 levels are associated with mortality among hemodialysis (HD) patients. However, the pathophysiological role of FGF-23 in those patients remains unclear, so the association between serum FGF-23 levels and known cardiac biomarkers or echocardiographic measurements were investigated in long-term HD patients without cardiac symptoms. Methods and Results: The 87 consecutive patients treated in a single HD center (51 males, 36 females; mean age 64 years, mean HD duration 5.8 years) were included in this study. Comprehensive echocardiography was performed after HD. Blood samples were obtained before HD. Serum FGF-23 levels in dialysis patients were 1,171±553pg/ml. In univariate analysis, serum phosphate (r=0.443, P<0.001) and calcium levels (r=0.256, P=0.04), left ventricular mass index (LVMI) (r=0.268, P=0.039) were significantly associated with FGF-23 levels. Neither the B-type natriuretic peptide (BNP) nor the cardiac troponin T level was correlated with FGF-23. In multivariate regression analysis, only LVMI (β=0.287, P=0.031, confidence interval (CI) 0.390-8.040) and phosphate levels (β=0.419, P=0.001, CI 57.12-207.7) and calcium levels (β=0.277, P=0.025, CI 24.95-360.1) remained significantly correlated with FGF-23. Conclusions: Beside BNP, FGF-23 was identified as a factor that is significantly associated with LVMI. FGF-23 could be a novel biomarker of left ventricular overload, which is closely associated with the increased risk of death in HD patients. (Circ J 2010; 74: 2734-2740)