The biological importance of iodine is due to the fact that it is essential for thyroid hormone production; the tragedy of its deficiency lies in the easily preventable, but largely irreversible, brain damage that occurs during fetal growth and infant development. Endemic goitre is the most visible manifestation of environmental iodine deficiency; other, more severe, effects included retarded fetal brain development, spontaneous abortion, stillbirths and infant deaths. A deficiency of iodine later in infancy and childhood causes mental retardation, delayed motor development, growth failure and stunting, lethargy, neuromuscular disorders (including squint and paralysis), and speech and hearing defects. In 1993 at least 1572 million people worldwide were at risk of iodine deficiency disorders (IDD), i.e. living in areas with goitre rates above 5%. At least 655 million of this number are affected by goitre, while an estimated 43 million people are affected by some degree of IDD-related brain damage. To prevent IDD, WHO recommends universal salt iodization. Iodized oil is the preferred interim measure in areas of moderate or severe deficiency that will not soon be covered by iodized salt. IDD is known to be a significant public health problem in 118 countries. There has been progress recently. While only 46 had national salt-iodization programmes in 1990, their number has increased in 1995 to 83. Jim Akre, Nutrition unit, WHO, Geneva