Hyperglycemic hyperosmolarity is part of a clinical spectrum of severe hyperglycemic disorders ranging from pure hyperglycemic hyperosmolarity without ketosis to diabetic ketoacidosis, with significant overlap in the middle. From 50 to 75 percent of hospitalizable patients who have uncontrolled diabetes present with significant hyperosmolarity. An altered state of consciousness attributable to uncontrolled diabetes is virtually always the result of severe hyperosmolar hyperglycemia. The linchpin of therapy is prompt, rapid administration of crystalloid solutions that have tonicity appropriate to the level of hyperosmolarity. A decrease in the plasma glucose concentration indicates the adequacy of therapy, especially rehydration; the goal is for the plasma glucose level to decline by at least 75 to 100 mg per dL (4.2 to 5.6 mmol per L) per hour. Patients with hyperosmolar hyperglycemic syndrome are often chronically ill, and they may have major total body deficits of potassium, phosphate and magnesium, as well as B-complex vitamins (especially thiamine). These deficits also require attention and correction during therapy.