Hyperglycemic hyperosmolar nonketotic syndrome (HHNS) was infrequently diagnosed till recently. Now it is being diagnosed with increasing frequency in obese children with type 2 diabetes mellitus (T2 DM) and its incidence is likely to go up, given global increase in incidence of childhood obesity, increased insulin resistance, and T2 DM. The syndrome is characterized by severe hyperglycemia, a marked increase in serum osmolality and dehydration without accumulation of β-hydroxybutyric or acetoacetic ketoacids. Significant ketogenesis is restrained by the ability of the pancreas, to secrete small amount of insulin. Prolonged phase of osmotic diuresis leads to severe depletion of body water, which excees that of sodium, resulting in hypertonic dehydration. These children, usually obese adolescents with T2 DM, present with signs of severe dehydration and depressed mental status but continue to have increased rather than decreased urine output and are at increased risk of developing rhabdomyolysis and malignant hyperthermia. Emergency treatment is directed at restoration of the intravascular volume, followed by correction of deficits of fluid and electrolyte (Na+, K+, Ca++, Mg++, PO4++), hyperglycemia and serum hyperosmolarity, and a thorough search for conditions that may lead to this metabolic decompensation and their treatment. Use of iso-osomolar isotonic fluid (0.9% saline) until hemodynamic stabilization initially, followed by 0.45% saline with insulin infusion at the rate of 0.1 units/kg/hour, addition of 5% dextrose in fluids and reduction of insulin infusion once the blood glucose is 250 to 300 mg/dl is generally recommended. However, evidence-based guidelines about composition and tonicity of fluids and electrolyte solutions for early resuscitation and rehydration, the rate of infusion—rapid vs slow, and insulin dose—low vs normal, in treatment of HHNS in children are awaited. Careful monitoring of glucose levels and ensuring adequate hydration in patients ‘at risk’ of HHNS, including those receiving medications that interfere with the secretion or effectiveness of insulin should decrease the risk of HHNS.