A girl with failure to thrive in the neonatal period was brought to the hospital at 10 weeks of age following a respiratory arrest, preceded by 12 h of vomiting and diarrhea. There was significant acidosis with a blood lactate of 8.8 mM. A high carbohydrate diet decreased her acidosis. Episodes of acidosis, often associated with infections, and accompanied by progressive neurological deterioration, have continued for 18 months. The activity of pyruvate dehydrogenase from cultured skin fibroblasts was 24% of that from normal fibroblasts. The activities of α-ketoglutarate dehydrogenase and branched-chain keto acid dehydrogenase were also deficient. The activity of the dihydrolipoyl dehydrogenase component (E3) of PDH in skin fibroblasts was 5% of that in control cell lines. Limited studies performed on liver and muscle biopsy specimens showed E3 activity in liver and muscle to be undetectable in both tissues. We conclude that the enzyme defect present in dihydrolipoyl dehydrogenase is responsible for the reduced activity of all three α-keto-acid dehydrogenase complexes and the patient's symptoms. Our results provide further evidence that the E3 component of these complexes is genetically and biochemically the same protein.