A severely mentally retarded infant with congenital lactic acidosis due to pyruvate carboxylase deficiency is reported. The patient suffered from vomiting and convulsions soon after birth and developed severe mental and motor retardation at 3 months of age. The persistent elevation of pyruvate and lactate in both blood and cerebrospinal fluid and hyperalanaemia suggested an impairment of pyruvate oxidation. The enzyme activities of pyruvate carboxylase in both liver tissues and cultured skin fibroblasts of the patient revealed values of about 5% of controls. However, pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase activities in liver tissues were within normal limits. The patient had no response to administration of large doses of thiamine, lipoic acid and biotin, clinically and biochemically. A prenatal diagnosis was performed in the second pregnancy and the pyruvate carboxylase activities of the cultured amniotic fluid cells obtained by amniocentesis were within normal limits.