The author describes severe hypernatremic dehydration in a 12-day-old, breast-fed infant. The mother's breast milk sodium level at the 13th day of the infant's life was 47 mEq/L, more than three times normal value. At the 30th day, the mother's breast milk sodium level was 16 mEq/L, still more than twice the normal value. To the author's knowledge, this is the first time in a case of this type that the breast milk sodium value of the mother has been followed this late in the infant's life. Results are consistent with the hypothesis that delayed maturation of breast milk composition is of central importance in this condition. A search of the literature reveals that the mothers of these babies fit a clinically distinct profile. Elevated breast milk sodium with hypernatremic dehydration is a cause of significant morbidity in some breast-fed infants. The pathogenesis of this condition and the implications for its prevention are discussed.