Disorders resulting from inborn errors of metabolism (IEM) affect very small numbers of individuals. The entire population, however, of patients suffering the results of inherited metabolic disorders is large, and has been of increasing concern to patient groups and health care professionals in the United States as well as other countries throughout the world. The 1983 US Orphan Drug Act (ODA) serves to facilitate the development of drugs to treat rare diseases by providing several economic incentives. The sponsor of a product designated as an orphan by the Food & Drugs Administration (FDA) Office of Orphan Products Development (OPD) qualifies for tax credits on clinical trial expenses, the award of grant funding by FDA, through the OPD, and 7 years of marketing exclusivity for a designated drug, or biological product that receives FDA market approval. Orphan drug legislation in the US has benefited victims of IEM by encouraging development of drugs for metabolic deficiencies affecting populations that otherwise would be ignored. America's solution to the orphan drug problem has had worldwide impact. The success of this legislation was a factor leading to the 1993 orphan drug law in Japan; the 1997 implementation of a process whereby most FDA-approved orphan drugs and biological products will be similarly approved in Australia; and, in 1999, regulation on orphan medicinal products in the European Union (EU). Today, international support for rare disease research is providing stimulus and motivation to overcome the financial barriers and encourage development of treatment for very rare diseases throughout the world.