Carnitine plays an essential role in the transfer of long-chain fatty acids across the inner mitochondrial membrane, in the detoxification of acyl moieties, and in maintaining normal levels of free coenzyme A. Although carnitine can be synthesized in liver and kidney, normal adults obtain the majority of carnitine from the diet. Preterm newborns have a reduced capacity to synthesize carnitine. Total parenteral nutrition lacks carnitine and exposes very low birth weight infants to carnitine deficiency, with decreased production of ketones from long-chain fatty acids. Supplementation with low doses of carnitine improves nitrogen balance and growth in these infants. Carnitine deficiency can be part of a number of inherited and acquired diseases. Primary carnitine deficiency is an autosomal recessive disorder characterized by increased losses of carnitine in the urine and decreased accumulation in the heart and skeletal muscle caused by defective carnitine transport. This condition is corrected by high-dose carnitine supplementation. Secondary carnitine deficiency can be caused by increased losses, pharmacological therapy, or a number of inherited metabolic disorders that must be correctly diagnosed before initiating carnitine supplementation.