SummaryThe most common defect associated with deficiency of the pyruvate dehydrogenase (PDH) complex occurs in the E1 component, specifically due to mutations in the X‐linked E1α gene. Clinical sequelae of these mutations, which range from severe neonatal lactic acidosis to carbohydrate‐sensitive ataxia, can be different in males and females depending on the nature of the mutation and, in the case of females, on the X‐inactivation pattern in different tissues. Males have a high representation of missense mutations among the patient cohort, while females are much more likely to have DNA rearrangements, particularly toward the 3′ end of the coding sequence of the gene. Missplicing mutations involving exon 6 deletion have been reported, as has a missense mutation conferring true thiamin‐responsiveness of the enzyme and the patient's clinical symptoms.Pyruvate carboxylase deficiency, on the other hand, is a true autosomal recessive disease, though it has high occurrences in particular ethnic groups, especially in Algonkian‐speaking Amerindians and in Arabs. In the former group the defect is a simple type in which material cross‐reactive to pyruvate carboxylase antibody is present in cultured cells (CRM+ve). In the latter group, cross‐reacting material is rarely present (CRM−ve). The CRM+ve patients can survive into teenage years with careful supervision, while the CRM−ve patients have complications due to hyperammonaemia and dysfunction of the urea cycle and rarely survive beyond 3 months of life.