The driving force in the early recognition and treatment of metabolic emergencies is the maintenance of a suspicion that such a disorder may be the underlying cause. The classic presentation of inborn errors of metabolism is with a free period of apparent health that may last days or even years, but then followed by an overwhelming life-threatening disease. Metabolic decompensation is often precipitated by acute infection and its attendant catabolism. Catabolism may also be induced by fasting, injury, surgery, or medication. Such a patient may have cycles of acute illness precipitating admission to the hospital, cessation of feedings and administration of parenteral fluids and electrolytes with recovery and discharge only to repeat the cycle until the diagnosis is made and appropriate therapy initiated, or the patient dies undiagnosed in such an episode. Initial laboratory evaluation needs only the routine clinical laboratory to establish acidosis or alkalosis, hyperammonemia, ketosis, hypoglycemia, or lactic acidemia, and to detect acute organ dysfunction (e.g., acute renal failure, acute liver failure, cardiomyopathy, rhabdomyolysis, or pancreatitis). Appropriate tests must be ordered on critical samples retrieved during the acute phase before treatment. Success with treatable disorders is substantially dependent on rapid and appropriate intervention, and delay may lead to irreversible neurological damage or death.