A number of studies are investigating the role of n-3 polyunsaturated fatty acids in children with metabolic inborn errors, while the effects on visual and brain development in premature infants and neonates are well known. However, their function incertain chronic neurological, inflammatory and metabolic disorders is still under study. Standards should be established to help identify the need of docosahexaenoic acid supplementation in conditions requiring a restricted diet resulting in an altered metabolism system, and find scientific evidence on the effects of such supplementation. This study reviews relevant published literature to propose adequate n-3 intake or supplementation doses for different ages and pathologies. The aim of this review is to examine the effects of long chain polyunsaturated fatty acids supplementation in preventing cognitive impairment or in retarding its progress, and to identify nutritional deficiencies, in children with inborn errors of metabolism. Trials were identified from a search of the Cochrane and MEDLINE databases in 2011. These databases include all major completed and ongoing double-blind, placebo-controlled, randomized trials, as well as all studies in which omega-3 supplementation was administered to children with inborn errors, and studies assessing omega-3 fatty acids status in plasma in these pathologies. Although few randomized controlled trials met the inclusion criteria of this review, some evidenced that most of children with inborn errors are deficient in omega-3 fatty acids, and demonstrated that supplementation might improve their neural function, or prevent the progression of neurological impairment. Nontheless, further investigations are needed on this issue.