Fifty years ago Switzerland was known to be an endemic goiter area. Iodine deficiency has since been abolished by supplementation of table salt with iodine and increasing consumption of imported food. On the basis of the large number of specimens in the University Institute of Pathology in Zürich, an attempt has been made to determine whether morphological aspects of malignant thyroid tumors have changed in the intervening perified according to the WHO nomenclature of 1974. Comparison of these results with those of a similar study covering the period from 1962 and 1973 showed that the relative incidence of some tumor types definitely changed following the introduction of iodine prophylaxis. Follicular and anaplastic carcinomas have decreased in number, while papillary carcinomas are nowadays more frequent. The sex-ratio distribution has also been modified, female patients being now in the majority. Altogether the proportion of differentiated carcinomas has increased during the period from 1962 onwards, and their far more favourable prognosis will probably lead to retrogressive mortality rates for malignant thyroid tumors.