Congenital lactic acidosis is defined by the presence of a metabolic acidosis due to the accumulation of lactic acid in blood. Congenital defects of any one of the multiple enzymatic steps of pyruvate utilization induce accumulation of pyruvate and lactate, but usually to levels that do not provoke metabolic acidosis. Lactic acidosis is therefore an extreme situation, due either to very severe defects or to acute metabolic crisis associated with less severe defects. It occurs mostly in neonates or very young infants, with polypnea, severe hypotonia, lethargy, and vomiting, after a silent period during which the children were considered as normal. Facial dysmorphism and cerebral malformations may be noted, as well as diverse organ involvement such as hypertrophic myocardiopathy, tubulopathy, or liver insufficiency.