In a randomized prospective study in 26 low birth weight infants (LBWI) small for gestational age (SGA) the concentrations of calcium and phosphorus as well as the activity of alkaline phosphatase in serum were measured up to the 42nd day of life. In addition the renal excretions of calcium and phosphorus were estimated in 12 of these infants. 42 very low birth weight infants (VLBWI) and 83 LBWI, both appropriate for gestational age (AGA), were included in the study as controls. There are changes in direction of lower concentrations of phosphorus and higher activities of alkaline phosphatase in the serum of SGA infants if compared to the control groups, but all values were within the normal range and all differences between the study groups are not significant. In contrast to this small differences of the serum parameters the renal excretion of phosphorus is significantly lower and the excretion of calcium significantly higher in the SGA group than in the control groups even on the third day of life. In the VLBWI, AGA such signs of a phosphorus deficiency appear later. The results suggest that infants born as SGA are not sufficiently provided with phosphorus during the intrauterine development. Thus, phosphorus supplementation has to be started immediately after birth in SGA infants. The measurement of renal excretions of calcium and phosphorus is more effective than that of serum parameters for monitoring the calcium-phosphorus-homöostasis during the first weeks of life.