Stupor in patients with nonketotic hyperglycemia has been ascribed to hyperosmolarity, but the cause of depressed consciousness in patients with ketoacidosis has been puzzling. In this study, blood pH, serum glucose and sodium concentrations, and serum osmolality were measured in eighty-five consecutive episodes of diabetic ketoacidosis and forty-seven of nonketotic hyperglycemia. In the acidotic patients, as in those with nonketotic hyperglycemia, stupor closely paralleled hyperosmolarity and not the severity of acidemia. Indeed, the mean elevations of serum osmolarity were almost the same in the ketotic and in the nonketotic patients who were deeply obtunded. It seems likely that depression of consciousness in patients with severely uncontrolled diabetes mellitus, if not due to a nonmetabolic disorder, such as acute stroke, is attributable to hyperosmolarity, whether or not ketoacidosis is present.