Aim: To identify neonatal weight loss and the incidence of hypernatremia in term, healthy breast-fed infants during the first 3 days of life and the effect of possible maternal and/or neonatal factors that interfere with successful breast-feeding.Methods: Term and exclusively breast fed infants born between January 2004-June 2004 in Istanbul Medical Faculty, Department of Obstetrics and Gynecology were evaluated retrospectively.Results: A total of 460 term neonates met our selection criteria of which 57 % were born vaginally and 55 % were male. Mean gestational ages were 38 ±1 (37-42) wks, mean birth weight 3,309±418 (2,230-4,500) g. Mean ages of the mothers were 28 ±5 (17-47) yrs and 53 % of them were primiparous. Mean percentage of weight loss during the first 24 hrs, 48 hrs and 72 hrs were 2.7 % ±1.7 (0-9.67), 4.6 % ±2.0 (0-10.7) ve 4.7 % ±2.4 (0-10.6) respectively. Maximal weight loss was during the first 24 hrs. The infants of primiparous mothers and caesarean section-delivered neonates had an increased weight loss during 48 hrs (p<0.05). In 124 of 460 (27 %) babies serum sodium concentration was measured because of pathological weight loss. In 32 (26%) of 124 neonates serum sodium concentration exceeded 149 mEq/L .Maximal serum sodium concentration was 154 mEq/L.Conclusion: All breast-fed infants should be weighed during the first three days and if there is a pathological weight loss serum sodium concentration should be measured. A careful breast-feeding assessment and breast-feeding support are effective methods to prevent pathological weight loss and hypernatremia during first three days of life in exclusively breast-fed infants.