In 12 diabetic patients who were being treated with maintenance hemodialysis or maintenance peritoneal dialysis, coma and other neurologic deficits did not occur in spite of extremely elevated serum glucose levels. The mean serum values of these patients were: glucose 1,174 +/- 248 (SD) mg/100 ml, sodium 125 +/- 5 mEq/l, calculated total osmolality 342 +/- 13 mOsm/kg water and calculated effective osmolality (without urea) 316 +/- 13 mOsm/kg water. It is suggested that the absence of osmotic diuresis and the lack of substantial osmotic ultrafiltration prevented the development of hypernatremia and marked hyperosmolality. The osmolar effect of glucose alone at these serum concentrations apparently was not sufficient to induce neurologic impairment.