several fatty acid oxidation disorders were targeted for inclusion in newborn screening programs. Medium chain acyl-CoA dehydrogenase deficiency (MCADD), which had been implicated in several cases of sudden infant death syndrome was one of the first conditions targeted for inclusion. MCADD was the first condition added when the United Kingdom expanded their screening program from PKU only. Population based studies in Germany, the United States and Australia put the combined incidence of fatty acid oxidation disorders at 1:9300 among Caucasians. The United States screens for all known fatty acid oxidation disorders, either as primary or secondary targets, while other countries screen for a subset of these. The introduction of screening for fatty acid oxidation disorders has been shown to have reduced morbidity and mortality associated with the conditions, particularly MCADD. An Australian study found a 74% reduction in episodes of severe metabolic decompensation or death among individuals identified by newborn screening as having MCADD versus those who presented clinically prior to screening. Studies in the Netherlands and United Kingdom found improvements in outcome at a reduced cost when infants were identified before presenting clinically. Newborn screening programs have also expanded the information base available about some rare conditions. Prior to its inclusion in newborn screening, short-chain acyl-CoA dehydrogenase deficiency (SCADD) was thought to be life-threatening. Most patients identified via newborn screening as having this enzyme deficiency were asymptomatic, to the extent that SCADD was removed from screening panels in a number of regions. Without the cohort of patients identified by newborn screening, this clinical phenotype would likely not have been identified. Endocrinopathies The most commonly included disorders of the endocrine system are congenital hypothyroidism (CH) and congenital adrenal hyperplasia (CAH). Testing for both disorders can be done using blood samples collected on the standard newborn screening card. Screening for CH is done by measuring thyroxin (T4), thyrotropin (TSH) or a combination of both analytes. Elevated 17α-hydroxyprogesterone (17α-OHP) is the primary marker used when screening for CAH, most commonly done using enzyme-linked immunosorbant assays, with many programs using a second tier tandem mass spectrometry test to reduce the number of false positive results. Careful analysis of screening results for CAH may also identify cases of congenital adrenal hypoplasia, which presents with extremely low levels of 17α-OHP. CH was added to many newborn screening programs in the 1970s, often as the second condition included after PKU. The most common cause of CH