DKA and HHS represent two extremes in the spectrum of decompensated diabetes mellitus. Their pathogenesis is related to absolute or relative deficiency in insulin levels and elevations in insulin counterregulatory hormones that lead to altered metabolism of carbohydrate, protein, and fat and varying degrees of osmotic diuresis and dehydration, ketosis, and acidosis. In DKA, insulin deficiency and ketoacidosis are the prominent features of the clinical presentation, and insulin therapy is the cornerstone of therapy. In HHS, hyperglycemia, osmotic diuresis, and dehydration are the prominent features, and fluid replacement is the cornerstone of therapy. As many as one-third of patients may have mixed features of both DKA and HHS. Because the three-pronged approach to therapy for either DKA or HHS consists of fluid administration, intravenous insulin infusion, and electrolyte replacement, mixed cases are managed using the same approach. The therapeutic regimen is tailored according to the prominent clinical features present. In adult patients with mixed features, fluids may be administered more rapidly than they would be in younger patients, or in patients with DKA alone, because the risk for fatal cerebral edema in adults is low and the consequences of undertreatment include vascular occlusion and increased mortality. In younger patients with mixed features, rapid correction of metabolic abnormalities and, consequently, of hyperosmolarity by administration of hypotonic fluids and insulin should be avoided to decrease the risk for precipitating cerebral edema. In addition, if ketoacidosis has been a prominent feature in a mixed case, the patient may have type 1 diabetes with no residual pancreatic islet beta cell secretion and may subsequently need ongoing, life-long insulin therapy after resolution of the acute episode of decompensated diabetes. ICU admission is indicated in the management of DKA, HHS, and mixed cases in the presence of cardiovascular instability, inability to protect the airway, obtundation, the presence of acute abdominal signs or symptoms suggestive of acute gastric dilatation, or if there is not adequate capacity on the floor unit to administer the intravenous insulin infusion and to provide the frequent and necessary monitoring that must accompany its use.