Published reports100,102,108 indicate that the degree of success attained in the treatment of infants with urea cycle disorders has not been completely satisfactory. Some new approaches in dietary management, using a combination of arginine, benzoate, and phenylacetate may be useful,67,72,76 but more studies are required. In many instances, clinical problems reflect delays in biochemical diagnosis. Families at risk need to be identified and the genotype of the fetus should be established as soon as feasible after conception, so that appropriate management can be provided during pregnancy and after delivery. Availability of effective methods for carrier detection is a critical component of this process. With any metabolic disorder, a number of factors should be considered for carrier detection. These include: (1) frequency of occurrence of affected patients in the available population, (2) mode of inheritance, (3) cost-benefit ratio, (4) soundness of the methodology used, and (5) feasibility of prenatal diagnosis. Interestingly, the Tay-Sachs prevention program with the major objective of carrier identification is the only accepted program that has been widely discussed and evaluated.109 The frequency of occurrence of each of the five urea cycle enzyme disorders is uncertain, but ornithine transcarbamylase (OTC) deficiency, the only sex-linked disorder, is considered to be the most common. Not only the hemizygotes, but also many of the heterozygotes of OTC deficiency are clinically affected. The frequency of argininosuccinic aciduria has been estimated to be 1 in 60,000, based on results of a newborn screening program carried out in Massachusetts.110 Carrier identification of urea cycle disorders in the general population is not practical because of cost considerations, but it should be applied to members of affected families.