Introduction & Objective: Hypernatremia is associated with serious complications in infants. Given the high morbidity and mortality of neonatal hypernatremia, and since some complications can be prevented, this study was aimed to examine the prevalence of hypernatremia in hospitalized infants and related risk factors.Materials & Methods: This cross-sectional study was performed on 1923 neonates hospitalized in Bahrami Hospital affiliated to Tehran University from October 2011 to October 2012. Demographic data , ward ,gender ,mode of feeding ,gravidity, admission season and method of delivery related to the infants were recorded. For infants with a serum sodium above 150 meq/lit, additional information were collected, including serum urea level, gestational age, date of birth, birth weight, admission weight, head circumference and clinical symptoms. Finally, the data were analyzed using the SPSS software.Results: 74 out of 1923 neonates had serum sodium above 150 meq per liter, which their mean Na was significantly associated with their age groups (P=0.04) and weight loss (P=0.008). Furthermore, the mean of urea in these neonates was significantly related to weight loss (P<0.001). The prevalence of hypenatremia in the hospitalized neonates was 5.2% in autumn, 4.9% in winter, 1.8% in spring and 3.4% in summer.Conclusion: The common symptoms of hypernatremia, icterus and weight loss often cannot be evaluated due to late referral (> a few days after birth) or to the non-exact weight at the birth time (risk factors). The study results confirmed that weighing the neonates at 72 to 96 hours after birth can prevent hypernatremia in infants.