We describe the clinical, biochemical, and molecular characteristics of 31 patients with hepatic respiratory chain deficiencies to suggest possible guidelines for a liver biopsy. Initially, 67% of the children did not have any sign of hepatic dysfunction, and 35% presented exclusively with neurologic symptoms. Initial hyperlactacidemia was severe in 52%. Mortality was high (52%) and more marked in newborns; 28% never developed hepatic disease over time despite long-term follow-up. Hepatic, nonspecific multisystem initial symptoms, and constant hyperlactacidemia had significant statistical value as negative prognosis factors. We conclude that liver biopsy should be considered not only in patients with hepatic involvement, but also in patients with predominant neurologic disorders if there is a suspicion of a mitochondrial respiratory chain defect.