In certain regions of Switzerland, before prophylaxis, 0.5% of the inhabitants were cretins, almost 100% of schoolchildren had large goitres and up to 30% of young men were unfit for military service owing to a large goitre. Iodization of salt was introduced in 1922 at 3.75 mg I per kg and the iodine content was doubled twice, in 1962 and 1980, to the present 15 mg I per kg. In 1988, 92% of retail salt and 76% of all salt for human consumption (including food industry) was iodized, even though its use is voluntary. Urinary iodine excretion, previously between 18 and 64 μg per per day, has now risen to 150 μg per day. No new endemic cretins born after 1930 have been identified. Goitre disappeared rapidly in newborns and schoolchildren, more slowly in army recruits, and incompletely in elderly adults. In some Cantons (by constitution in charge of health matters and the salt monopoly) which allowed iodized salt only in 1952, disappearance of goitre lagged behind accordingly, proof that iodized salt was the cause of regression. The Swiss data provide evidence that isolated deafness, mental deficiency, and short stature, each without the other attributes of cretinism have also decreased. Adverse effects of iodized salt were minimal, possibly because the initial iodine content of salt was chosen very low. Iodization of salt has proved a highly cost-effective preventive measure in Switzerland.