We studied the changes in serum sodium (Na) and potassium (K) levels in seventeen patients in diabetic ketoacidosis and nine patients in non-ketotic hyperosmolar coma, who had marked hyperglycemia (707.4 +/- 75.6 mg/dl, mean +/- SEM) and dehydration. The disorder characterized two types of alteration. The one group was hyponatremia with hyperkalemia in 17 patients in diabetic ketoacidosis (132.9 +/- 2.0 and 5.7 +/- 0.2 mEq/l), and 4 patients in non-ketotic hyperosmolar coma (125.8 +/- 4.3 and 5.2 +/- 0.5 mEq/l). The other was hypernatremia (162.5 +/- 1.8 mEq/l) with hypokalemia (3.4 +/- 0.2 mEq/l) in 5 patients in non-ketotic hyperosmolar coma. Intensive therapy with insulin and fluid administration improved the diabetic hyperglycemia and associated abnormalities. The vectors showing the normalization of serum Na and K levels was in quite opposite directions between the patients with hyponatremia with hyperkalemia and those with hypernatremia with hypokalemia. The amounts of loss of circulatory blood volume exceeded 20% in three groups of patients, a loss greater in the hypernatremic patients than in the hyponatremic ones. These results indicate that serious body water depletion produces hypernatremia instead of hyponatremia in patients in diabetic coma. The disorder may be caused by the altered distribution of electrolytes between the intra- and extra-cellular spaces.