A ten-month-old infant with pyruvate dehydrogenase deficiency received anaesthesia on two occasions, once for a laparotomy and once for a tracheostomy. During both anaesthetics (different techniques) she developed an increase in arterial lactate levels and a metabolic acidosis. Pyruvate dehydrogenase deficiency results in the inability to metabolize pyruvate with resultant accumulation of pyruvate and lactate. Inhibition of gluconeogenesis, which may be produced by halothane and thiopentone, will also increase lactate levels. Other causes of increased lactate levels are hypocarbia and high carbohydrate intake. In this patient hypocarbia may have produced increased lactate levels and increased the metabolic acidosis. Recommendations include avoidance of halogenated anaesthetics, avoidance of lactate containing solutions, maintenance of normocarbia, and stress-free anaesthesia.