Preterm infants are more prone to bone mineral deficiency the lower their birth weight. To achieve the intrauterine bone mineral accretion rate postnatally, 74 low-birth-weight infants (median birth weight, 980 g; range 430–1.580 g) were each supplemented enterally and/or parenterally with calcium and/or phosphorus in gradually increased amounts. The aim was to yield a simultaneous urinary excretion of Ca and inorganic phosphorus (Pi) at low concentrations (1–2 mmol/L) in spot urine specimens taken twice weekly. The hypothesis was that the intrauterine mineralization rate (4.5 mg cm−1/100 g weight gain) would be achieved postnatally in very low-birth-weight infants, if they were supplemented with enough Ca and/or Pi to effect at least a low (1–2 mmol/L) simultaneous urinary excretion of both ions, as compared with infants who do not excrete both ions and would accrete the bone minerals at a lower rate. The change in bone mineral content was measured by single photonabsorption densitometry and related to weight gain during periods of 2 to 6 wk. Infants who simultaneously excreted Ca (>1.2 mmol/L) + Pi (>0.4 mmol/L) in more than half of the urine samples retrospectively showed the highest bone mineral accretion, 5.1 mg cm−1/100 g weight gain, which was equivalent to the fetal mineralization rate (4.5). In this group the bone mineral status significantly contributed to the variance of the bone mineral accretion rate; severely demineralized infants showed a catch-up mineralization. A significantly lower rate (2.4) was observed in infants who excreted Ca + Pi in less than half of the urinary samples. Supplementation with Ca and Pi up to the point where both ions are simultaneously excreted with the urine at concentrations >1.2 and >0.4 mmol/L, respectively, offers a simple and safe way of achieving the fetal bone mineral accretion rate in preterm infants, taking into account their individual needs, the varying mineral content of breast milk, and the different compositions of special-care formulas. A further prospective study is needed to test the hypothesis.