To evaluate the age-dependent phosphate homeostasis, we studied the serum inorganic phosphate (P-i) concentration in 78 recipients, aged 5-25 years, a year after renal transplantation (RT). The significant age-dependent decline of the serum P-i concentration was observed in recipients (p < 0.0001) as well as in normal children. Our study revealed that a circulating factor may play a central role in the age-dependent change of phosphate regulation in human. Copyright (C) 2002 S. Karger AG, Basel.