The objective of this study was to determine the specific reference values for urinary calcium/creatinine (UCA/Cr) (mg/mg) in healthy breast-fed newborns, and to evaluate the relationship between UCa/Cr, urinary sodium/creatinine (UNa/Cr), urinary potassium/creatinine (UK/Cr) and UNa/UK ratios in the same group. A total of 88 infants aged between 0-28 days were enrolled in this study. They were divided into two age groups as follows: Group I: < or = 7 days of age; Group 2 infants aged between 8-28 days. Non-fasting spot urine was analyzed for Ca, Na, K and Cr. Significant differences were observed between the two groups in terms of UCa/Cr (0.11+/-0.10 vs 0.27+/-0.23, p<0.001), UNa/Cr (1.29+/-1.63 vs 5.5+/-4.83, p<0.001), and UK/Cr (0.94+/-0.99 vs 2.82+/-2.3, p<0.001). The data showed positive correlation between UCa/Cr and age (r=0.38, p<0.001) as well as between age and UNa/Cr ratio (r=0.68, p=0.0001) and between age and UK/Cr ratio (r=0.57, p<0.0001). Additionally, there was a positive correlation between UNa/UK and age (r=0.40, p=0.001). The UCa/Cr ratio positively correlated with UNa/Cr whereas no correlation was found between UCa/Cr and UNa/Uk ratio. Our data suggest that the healthy neonates differ from the hypercalciuric patients by exhibiting a linear correlation between Na/K and UCa/Cr. As the normal values of UCa/Cr, UNa/Cr, UK/Cr, UNa/UK ratios in the early neonatal period differ from those in the late neonatal period, these differences should be taken into consideration when assessing urinary excretion of these parameters for diagnostic purposes in the early and late newborn periods.