In a study based on a review of all thyroid function tests performed in the Funen region in Denmark over a 3 yr period, from Jan. 1, 1972 to Dec. 31, 1974, the annual incidence rates of thyrotoxicosis were 46.5/100,000 for women and 8.7/100,000 for men. In both sexes the age-specific rates rose with age. An analysis of the geographical distribution of the cases showed a significantly lower incidence rate among women from an urban than from rural areas. The life-time risk of developing thyrotoxicosis is approximately 5% for women and 1% for men; the prevalence rate of existing and previously diagnosed thyrotoxicosis is 1.4% for Danish women and 0.3% for Danish men.