FIVE CASES of hypernatremia (serum sodium levels above 150 mEq. per liter) associated with brain injury were reported by Allott1 in 1939. Despite numerous subsequent reports, the frequency with which hypernatremia occurs has not been fully appreciated. Nor has our understanding of the mechanisms producing hypernatremia been greatly enhanced In the past four years we have encountered over 100 unequivocal instances of hypernatremia (hyperosmolarity) in acutely ill patients. It seems convenient to divide these cases into three general groups, to which we have arbitrarily given descriptive headings. These divisions are based in part on a concept of the mechanism producing the hypernatremia and in part on the clinical syndrome with which the hypernatremia is associated. Hypernatremia has been said to be the result of severe dehydration. In the experimental subject prolonged water deprivation will not produce significant hyperosmolarity.2 It is necessary to add marked water loss to achieve