Urinary calcium/creatinine ratio (U Ca/creat) was studied in infants, term and preterm newborn babies. In 31 1.5-24 months old healthy infants, the median U Ca/creat was 0.16 mmol/mmol (range 0.013-1.17) and was similar to the value obtained in children older than 4 years. In 55 healthy full-term newborns studied in the first week of life, the median U Ca/creat was 0.12 mmol/mmol. However the range of values was extremely wide (0.0006-4.75), suggesting that the U Ca/creat ratio is of little interest to screen for hypercalciuria during the neonatal period. In 31 premature newborns, the median U Ca/creat was 1.08 mmol/mmol, a value significantly higher than in the two other groups (P < 0.001); as in the term newborns there was a very wide range of values (0.057-6.83). However after excluding the premature babies with elevated serum 25 OHD level, this difference was not statistically significant.