A balance study of Ca and P has been performed in 12 Very Low Birth Wt babies receiving prolonged Total Parenteral Nutrition. The mean intake of both minerals was 54.4 mg/kg/day (range 40-70). In order to avoid the formation and precipitation of CaP crystals in the solution, fructose-1,6-diphosphate was used as a source of P. 30 balance studies were performed between the seven and 63 day of life: they were always positive with a mean retention of 47.4 mg/kg/day of Ca and 48.1 mg/kg/day of P. For both minerals, 88% of the amount infused was retained: the correlation between intake and retention was linear and statistically significative (Ca:r = 0.9, p < 0.0001; P:r = 0.68, p < 0.0001). The post-natal and post-conceptional ages of the babies had no influence on Ca and P balance. The blood levels of Ca and P were poorly correlated to both intake and excretion, and were not as indicative of the mineral balance as the retention rates calculated on the basis of the 24 h urinary excretion of the minerals. A very useful test for clinical monitoring of Ca and P balance proved to be the Ca/creatinine and P/creatinine ratios measured on simple urine samples, which were strongly correlated to 24 h excretion. All infants developed radiological signs of mild osteopenia, but there was no case of acute metabolic derangement or rickets. Our data demonstrated that even in sick VLBW infants on TPN it is possible to achieve good retention rates of Ca and P, which are not different from those observed in well VLBW babies fed a 'standard' premature formula.