BACKGROUND: Hypernatremic dehydration in neonates is a potentially devastating condition. Recent reports have identified breast-feeding malnutrition as a key factor in its pathophysiology. METHODS: Using a theoretical framework for breast-feeding kinetics, a retrospective chart review of all neonates less than 28 days of age who were seen at either British Columbia's Children's Hospital or the Vancouver Breastfeeding Centre between 1991-1994 was conducted to identify and classify possible causes of breast-feeding malnutrition among neonates who developed hypernatremic dehydration. RESULTS: Twenty-one cases hypernatremic dehydration were identified. Infant weight loss ranged from 8% to 30% of birth weight, and serum sodium levels ranged from 146 mmol/L to 207 mmol/L. In each case, maternal or infant factors (e.g., poor breast-feeding technique, lactation failure following postpartum hemorrhage and infant suckling disorders associated with cleft palate or ankyloglossia) that could interfere with either lactation or breast-feeding dynamics and account for insufficient breast milk intake were identified. INTERPRETATION: Prenatal and in-hospital screening for maternal and infant risk factors for breast-feeding malnutrition combined with early postpartum follow-up to detect excessive infant weight loss are important for the prevention of neonatal hypernatremic dehydration.