In the last few years, inborn errors of oxidative phosphorylation have been recognized as possible causes of hepatic failure in infancy and respiratory enzyme deficiencies have been described in several tissues of affected individuals. Here, we report on cytochrome c oxidase deficiency in the liver but not in the skeletal muscle of a 5-month-old girl who presented hepatic failure in early infancy. Persistent hyperlactatemia (> 4 mM, normal < 2.4) with high lactate/pyruvate (L/P) molar ratios in plasma, and their further elevation in the post-absorptive period were suggestive of an inborn error of oxidative phosphorylation. However, no mutation in the coding sequences of the liver-specific subunits of cytochrome c oxidase (VIa and VIIa) has been detected and no major rearrangement or depletion of the mitochondrial DNA has been observed. Based on this observation we suggest that inborn errors of oxidative phosphorylation be considered in the diagnosis of severe hepatocellular dysfunction of unknown origin, especially when an abnormal oxidation-reduction status is found in the plasma and even if normal respiratory enzyme activities are found in peripheral tissues. The findings of normal respiratory enzyme activities in skeletal muscle, circulating lymphocytes or cultured skin fibroblasts does not rule out this diagnosis. Instead, the negativity of these tests should prompt one to carry out the specific enzyme assays in the tissue which expresses the disease, namely the liver.