Fructose-1,6-bisphosphatase (FBP1) deficiency is characterized by episodic acute crises of lactic acidosis and ketotic hypoglycemia, manifesting as hyperventilation, apneic spells, seizures, and/or coma. Acute crises are most common in early childhood; nearly half of affected children have hypoglycemia in the neonatal period (especially the first 4 days) resulting from deficient glycogen stores. Factors known to trigger episodes include fever, fasting, decreased oral intake, vomiting, infections, and ingestion of large amounts of fructose. In untreated individuals, symptoms worsen progressively as continued catabolism leads to multiorgan failure (especially liver, brain, and later heart). Morbidity and mortality are high. Sepsis, blindness, and Reye syndrome-like presentation have been reported. In between acute episodes, children are asymptomatic. While the majority of affected children have normal growth and psychomotor development, a few have intellectual disability, presumably due to early and prolonged hypoglycemia.