VLBW infants often develop bone mineral deficiency but the intrauterine bone mineral accretion (BMA) rate (4.5 mg/cm 100 g weight gain) can be achieved postnatally if the mineral supplementation is increased up to the point that both calcium and phosphorus are simultaneously excreted with the urine. The wide range ol bone minoral accretion (-2.2 to 13.3) observed in a previous study prompted us to look for variables that contribute to this variation. Methods: From a longitudinal study ot 74 VLBW infants (birth weight range 430 - 1.580 g, median 970; gestational ago 24 - 33, 28 weeks), who roceived stepwise increased supplements of Ca/P, 37 three-weeks-periods wore selected when urino samples (2/week) contained both Ca and P. BMA during those poriods, measured by single photon absorption densitometry at the right humerus, was related to the bone minoral status (actual bone mineral content minus weight related 50th centilo BMC at birth) at the beginning of each period (Postnatal age range 1.8 - 28 weeks, median 11; body weight range 1455 - 5685 g, median 2380). Results: BMA was directly proportional to the degree of bone mineral deficiency. The highest BMA was found in the infants with tho lowest mineral statusConclusion: Osteoponic VLBW infants show catch-up mineralization when they are sufficiently supplemented with Ca/P.