Lactic acidosis can develop during linezolid use. When no other risk factors can be identified, it possibly reflects mitochondrial dysfunction induced by linezolid. This chapter examines the role of mitochondrial impairment in the etiology of linezolid-induced lactic acidosis, with a special emphasis on critically ill subjects who are at higher risk of death. Lactic acidosis in critically ill subjects commonly develops due to tissue hypoxia and thus mainly reflects lactate overproduction via accelerated glycolysis. Linezolid inhibits bacterial growth by interfering with bacterial protein synthesis, as do many other antibiotics. Insights into the mechanisms responsible for the therapeutic effects of oxazolidinones are important to understand the potential mitochondrial toxicity of these antibiotics. Intra-mitochondrial translation of messenger RNAs (mRNAs) into proteins relies on transfer RNAs (tRNAs) and ribosomal RNAs (rRNAs) that are encoded by mitochondrial DNA (mtDNA). mtDNA sequence polymorphisms can influence the individual response to specific drugs.