In 19 preterm infants fed a standard formula for prematures (calcium (Ca) 13.5 mmol/l; phosphorus (P) 12.9 mmol/l), biochemical parameters of blood, serum and urine were determined before and during supplementation with Ca-L-lactate (final Ca concentration 20 mmol/l). In 8 preterm boys Ca and P balance were evaluated in addition. During Ca supplementation, the serum Ca levels, urine pH (without supplement 6.31, with supplement 6.73), and calciuria (46 mumol/kg/d vs. 98 mumol/kg/d) were increased, and urinary P (1.05 mmol/kg/d vs. 0.65 mmol/kg/d) and net acid excretion (1.70 mEq/kg/d vs. 0.89 mEq/kg/d) were decreased. Balance studies showed increased net intestinal Ca absorption during supplementation (37% vs. 56%) as well as improved Ca (0.8 mmol/kg/d vs. 1.85 mmol/kg/d) and P retention (0.97 mmol/kg/d vs. 1.45 mmol/kg/d). These data show that increased Ca intake given to optimize the Ca:P ratio improves mineral retention in preterm infants fed a standard formula. Ca and P intake should be thoroughly balanced to avoid side-effects like hypercalciuria or high renal net acid excretion.