The Department of Health and the National Screening Committee are funding a new newborn screening service for the rare inherited disorder medium chain acyl-CoA dehydrogenase deficiency, otherwise known as MCADD. Screening commenced as a pilot programme on 1 March 2004 from six laboratories in England. An evaluation study following all positive cases of MCADD from this first two-year pilot period will continue for a further three years to measure the overall effectiveness of screening for MCADD in the UK. This article reviews the background to the introduction of the pilot programme and describes the features and complications of MCADD, including the diagnosis and management.