Fibroblast growth factor 23 (FGF23), a recently discovered phosphaturic substance playing a key role in genetic and oncogenic phosphate diabetes, is involved in the physiological regulation of phosphate metabolism. Moderate idiopathic phosphate diabetes (IPD) leading to male osteoporosis and diffuse pain resembling fibromyalgia has been described. The aim of our study was to define the role of FGF23 in the mechanism of IPD. The study concerned 29 patients with IPD, mean age 53 +/- 11 years, of whom 72% were men. Fifteen subjects without bone disease and with normal serum phosphate and calcium levels were used as controls. Phosphate diabetes was confirmed by phosphate reabsorption level <85% and phosphate reabsorption threshold (TmPO4/GFR) <0.83. Known causes of phosphate diabetes were excluded. Fasting level of FGF23, serum phosphate, 1-25(OH)2D3, and parathyroid hormone were measured in patients and compared with FGF23 and serum phosphate in healthy controls. Spinal and hip bone mineral density (BMD) were measured by osteodensitometry. Sixteen of 29 patients had diffuse pain, 10 had osteoporosis according to the World Health Organization criteria, and 11 had osteopenia. Serum phosphate was significantly lower in patients than in controls, but FGF23 levels did not differ. Compared to patients with normal bone status, patients with osteopenia and osteoporosis had significantly decreased FGF23 levels, whereas serum phosphate was identical in the two groups. In all patients, serum phosphate and FGF23 were positively correlated and FGF23 and 1-25(OH)2D3 were negatively correlated. FGF23 seems not be a cause of IPD, and the FGF23/phosphate/1-25(OH)2D3 axis appeared to be functional.