A large number of diseases, at least 70, involve the metabolism of organic and amino acids. Some diseases present as repeated episodes of acute metabolic decompensation, and the central nervous system (CNS) involvement is usually secondary to the accompanying acidosis, hypoglycemia, or hyperammonemia or to the elevated concentration of amino acids. Nevertheless, it is the secondary involvement of the CNS that is important and is the main problem requiring management. Another group of these disorders involves the CNS primarily, with the systemic disease being negligible. Such disorders are difficult to label at the time of initial encounter as either an organic or amino acid disorder because there are no episodes of acid-base disturbance or disturbance of glucose