It is estimated that 75% of the daily requirement of carnitine comes from dietary intake and infants and young children have not yet developed biosynthesis. Therefore, the effect of diet on homeostasis of carnitine is very large, and there is a high risk of carnitine deficiency in children with severe physical and mental disabilities and children who have undergone surgery if they are nutritionally managed for a long time with enteral nutrition preparations that do not contain carnitine or total parenteral nutrition (TPN). In addition, carnitine deficiency has been reported in infants who are allergic to milk and are fed with some allergen-free formula, only soybean protein-based infant food, strict vegetarians, and patients with anorexia. Various symptoms such as hypoglycemia, hyperammonemia, and cardiac dysfunction might be observed but sometimes these symptoms are overlooked. Several clinical studies and many case reports of these carnitine deficiency in Japan are introduced in this chapter.