Introduction: Since it can cause life-threatening complications in newborns, diagnosis and treatment of hypernatremic dehydration associated with inadequate breastfeeding is important. Materials and Methods: Records of exclusively-breastfed newborns 37-42 weeks with hypernatremic dehydration serum Na ≥150mEq/L admitted between 2006 and 2012 were reviewed retrospectively. Results: The mean gestational age, birth weight, weight loss, maternal age, and age at diagnosis of 26 newborns with hypernatremic dehydration were 38.8±1.1 weeks, 3292±458 gr, 13.5±5.5%, 27.6±4.9 years, and 3.9±3.5 days, respectively. The percentages of female patients, caesarean delivery, and primipar mothers were 57.6%, 61.6%, and 57.6% respectively. Admission complaints were fever 30.7% , poor feeding and jaundice 26.9% , restlessness and hypoactivity 7.6% . Hypernatremic dehydration frequency within first five days, in summer season, during hospitalization were 84.6%, 73%, and 42.3%, respectively. The mean serum BUN, creatinine, Na levels were found 45.6±64.1 mg/dl, 1.5±2.3 mg/dl, and 157±11.9 mEq/L, respectively. Of 26 mothers, 57.6% had received breastfeeding education and 84% had inadequate fluid intake. Among four patients with seizures, three had prerenal failure, one had renal failure requiring dialysis, and brain edema developed in one. Serum Na levels were higher in infants who were baby of primipar mother p=0.002 , born in another hospital p=0.012 , from young mothers p=0.035 , from mothers with no breastfeeding education p=0.007 , and with delayed hospital admission p