Background: Neonatal hypernatremic dehydration (NHD) is a dangerous condition that can lead to severe weight loss, renal impairment, and central nervous system complications. We aimed to evaluate the consequences of NHD in infants in their second year of life. Materials and Methods: This was a prospective case-control study in Ghaem hospital, Mashhad, Iran. Sixty-five healthy breastfed neonates (serum sodium concentration <150mmol/L) and 65 hypernatremic (serum sodium concentration 150mmol/L) neonates were followed up from 2008 to 2011. Maternal and neonatal factors were compared between the two groups together with their growth parameters, and developmental milestones (using Denver II developmental assessment scores) were assessed and compared in ages 6, 12, 18, and 24 months, respectively. Results: The weight of infants at 6 months of age was significantly different between the two groups (7,2641,089g vs. 7,596957g, p=0.009). Twenty-five percent of infants in the group who had developed NHD had a delay in development at 6 months of age, with corresponding values of 21% at 12 months, 19% at 18 months, and 12% at 24 months of age. Developmental delay was approximate to 0.3% for the control group at similar ages. The severity of hypernatremia was strongly correlated with poor developmental outcome at 6 months (p=0.001). Serum sodium concentration of neonates was 153-195mg/dL in the NHD group. Median peak serum sodium was 158 +/- 16 in case group and 141 +/- 9 in control group. Serious complications were cerebral edema (five cases), hemorrhage (five cases), and kidney stones (six cases). Hypernatremic dehydration has an adverse effect on child development especially in the first year of life, their prevalence decreases with advanced age. Growth problems are also present during their first year of life. The major signs and symptoms of infants with poor prognosis on admission were poor feeding (8 infants, 61.5%), seizure (3 infants, 23.1%), hyperthermia (1 infant, 7.7%), and lethargy (1 infant, 7.7%). Conclusions: NHD affects growth parameters and developmental milestones of children. Occasionally the child's weight gain was normalized by the end of first year of life; although developmental delay continued, its severity was reduced, with age.