A retrospective study was carried out on the incidence of thyroid cancer in Iceland from 1955 to 1984. During this 30-year period 406 cases of thyroid cancer were registered. The incidence of 9.5 for females and 3.4 for males per 100,000 per year is at least twice as high as in the other Nordic countries and among the highest incidence figures reported anywhere. A considerable increase in the reported incidence of thyroid cancer was noted around 1965. The mean size of the cancer nodules at diagnosis decreased at the same time and survival rates of patients improved. The incidence decreased again during the last 5 years of the study period. Mortality rates remained similar during this 30-year period. The survival rate corrected for intercurrent death was similar for both papillary and follicular carcinomas. All patients with anaplastic carcinomas died within one year of diagnosis. Cox's regression analysis with multiple covariates revealed that age at diagnosis, anaplastic and medullary history type as compared with papillary type, pathological evaluation of tumour extent, and calendar period of diagnosis had significant prognostic power. Sex and follicular vs papillary histology type were not significant prognostic factors.