Hyperthyroidism is a well-described cause of hyperphosphatemia. We aimed to clarify the physiological role of FGF-23 in serum phosphate homeostasis in patients with Graves' disease during the course of treatment for hyperthyroidism. The study group comprised 56 patients (45 for a cross-sectional study and 11 for a longitudinal study) with Graves' disease. For the cross-sectional study, patients were assigned, on the basis of their serum phosphate level, to a hypophosphatemia group (n = 14), a normophosphatemia group (n = 16), or a hyperphosphatemia group (n = 15). Serum FGF-23, calcium, phosphate, parathyroid hormone, and 1,25-dihydroxyvitamin D (1,25(OH)₂D) levels were compared between the 3 groups. For the longitudinal study, we assessed changes in these biochemical indices before and after antithyroid treatment. In the cross-sectional study, the serum FGF-23 level was significantly higher (P < 0.05) in the hyperphosphatemia group than in the other groups (61 ± 36 ng/L vs. 31 ± 22 ng/L and 30 ± 9 ng/L). In the longitudinal study, serum levels of FGF-23 decreased significantly (P < 0.05) from a high of 54 ± 12 ng/L before treatment to 29 ± 14 ng/L after treatment. In contrast, the serum 1,25(OH)₂D level increased significantly (P < 0.005) from 55 ± 22 pmol/L before treatment to 185 ± 76 pmol/L 3 months after treatment. Serum FGF-23 levels were positively correlated with serum phosphate levels (P < 0.0001) and negatively correlated with serum 1,25(OH)₂D levels (P < 0.0001). The significant positive correlation between serum levels of phosphate and FGF-23 indicates that FGF-23 may play an important role in serum phosphate homeostasis by its up-regulation in the hyperphosphatemic condition.