Elevated breast milk (BM) Na concentration is regarded as responsible for elevated Na intake. To verify the clinical significance of milk Na concentration, we studied the relationship between BM Na+ concentration and infants' daily Na+ intake, infants' daily BM intake (DBMI) and percentage weight loss (%WL) in healthy newborn infants. All mothers who gave birth to a single healthy infant, between February and March 2004 at the Obstetric Clinic of University of Messina (Italy), were invited to participate if they were willing to attempt to breastfeed exclusively. BM Na+ concentration, DBMI, Na+ intake and %WL were determined on the third day after delivery. Statistical analysis was performed by Spearman's correlation test, classification and regression trees and the generalised linear model. Of the 270 eligible mothers, 208 participated in the study. The results showed that on the third day postpartum BM Na+ concentration was 23·05 (sd 1·10) mmol/l, mean DBMI was 202 (sd 68·9) g/d, and mean Na+ intake was 4·36 (sd 0·22) mmol/d and 1·36 (sd 0·07) mmol/kg per d. BM Na+ concentration was inversely related to infant DBMI, and Na+ intake was directly related to infant DBMI and not to BM Na+ concentration. %WL was significantly correlated only to DBMI. In conclusion, the present data demonstrate, for the first time, that when lactogenesis is suboptimal, BM Na+ concentration is higher, but infants' Na+ intake is lower. Finally, the present data probably suggest that for the clinical assessment of breast-feeding, evaluation of milk intake remains the best method.