Introduction: Hypernatremia described as a serum sodium level higher than 145 mEq/L, is a relatively common electrolyte disorder in newborn period. Material and Method: A total of 17 cases followed for dehydration and having a serum sodium level of >145mEq/L at our hospital between January 2007 and December 2008 were included in this study. Mother’s age, number of gestation, route of delivery, birth weight, weight at presentation, postnatal age, complaints at admission, physical examination, and serum biochemical parameters were recorded from the files of the patients. Results: Among all patients, 8 (47.1%) were female and 9 (52.9%), male. Median birth weight was 3340 gr (range: 2600-4600 gr); median weight loss, 17.1% (10-30%), median postnatal age, 6.2 days (2-16 days); and age of the mother, 24.9 years (21-39 years). All the cases were breastfed. Median serum sodium level was 158 mEq/L (range: 149-192 mEq/L), and median urea level, 122 mg/dL (range: 18-428 mg/dL). Of these 17 newborns, 12 (70.6%), cases presented with feeding problems; 2 (11.8%) presented with jaundice, and 3 (17.6%) presented with irritability. Neurological findings were detected in 6 (35.3%) cases. Oligo-anuric and/or hypotensive cases received fluid replacement therapies after loading of fluid. Sodium decline rate was 0.5±0.2 mEq/hour (range: 0.29-0.83). Time to reach normal sodium levels ranged between 24 and 80 hours in our cases. While no pathology was detected in 3 of 6 patients underwent renal ultrasonography, dilatation in pelvicalicial system was found in 2 cases and bilateral medullar nephrocalcinosis was observed in one case. Two cases who underwent cranial imaging had bilateral thalamic hemorrhage. Conclusion: Hypernatremia, not an uncommon problem in the newborn, may cause serious complications. In addition to giving education to mothers about feeding, close monitoring of the cases in respect to weight loss during early neonatal period is important in prevention of hypernatremia.