Fibroblast growth factor (FGF)23 is a hormone that regulates serum phosphate and 1,25-dihydroxyvitamin D levels. Hyperphosphatemia is sometimes observed in patients with acromegaly while the detailed mechanism of this abnormal phosphate metabolism remains to be elucidated. We have measured FGF23 levels in 18 patients before and after the surgery for acromegaly. Serum GH, IGF-I and phosphate significantly decreased after the surgery. In addition, FGF23 also reduced by the surgery. These results indicate that deficient action of FGF23 is not the cause of deranged phosphate metabolism in patients with acromegaly.