Hyperosmolar hyperglycemic state (HHS) is a complication of diabetes mellitus, specifically type 2 diabetes, that occurs when blood sugar levels are very high for a long time. HHS is characterized by severe hyperglycemia, extreme dehydration, hyperosmolar plasma, and altered consciousness. Unlike diabetic ketoacidosis (DKA), HHS does not involve the presence of ketones. HHS is usually precipitated by an infection, myocardial infarction, stroke, or another acute illness. The primary symptom of HHS is altered consciousness varying from confusion or disorientation to coma, usually as a result of extreme dehydration with or without prerenal azotemia, hyperglycemia, and hyperosmolality. Other symptoms of HHS include increased thirst (polydipsia), increased volume of urination (polyuria), and increased hunger (polyphagia). HHS is a potentially life-threatening emergency, and if not treated right away, seizures, coma, or death may result. The goal of treatment is to correct the water loss, improve blood pressure, urine output, and circulation, and decrease blood sugar levels. Treatment involves giving fluids and potassium through a vein (intravenously) and insulin to lower blood sugar levels. People who develop HHS are often already ill, and the risk of death among those affected is about 15%. To prevent HHS, it is important to control type 2 diabetes and recognize the early signs of dehydration and infection.