Extraosseous calcification in hemodialysis (HD) patients consists mainly of biological apatite, idealized as hydroxyapatite. Other suggested calcium phosphates are octacalcium phosphate (OCP) and brushite, both known to be hydroxyapatite precursors. Whatever the mechanisms of mineral deposition are, these mechanisms are always required to produce a supersaturated state, and that state can be calculated from the solubility product (SP) of the relevant mineral. Supersaturation in relation to serum ionized calcium [Ca 2+] and total inorganic serum phosphate (Pi) under normal and hyperphosphatemic conditions has been calculated. While supersaturation with respect to hydroxyapatite and OCP always exists, and supersaturation with respect to Ca5 (PO4)3 (HCO3) . 4H2O is just above the limit, supersaturation with respect to brushite solely occurs under hy-perphosphatemic conditions. In order to avoid supersaturation with respect to brushite the maximum serum phosphate level al-lowed is 1.9 mmol/L (5.8 mg/dl) and the calcium-phosphate product (Ca x P) 4.5 (mmol/L)2 (56 (mg/dl))2 .