The cure rate of hyperthyroidism after 131I therapy was analysed in 4 473 patients, aged between 14 and 91 years, treated between 1951 and 1975. Despite unchanged treatment principles, the cumulative incidence of cure increased for each 5-year period. Within the first 4 months 34 per cent of the patients with diffuse thyroid glands treated during 1951-1955 were cured, as compared with 59 per cent of those treated 1971-1975, i.e. an increment by a factor of 1.7. The corresponding figures for patients with nodular thyroid glands were 23 and 54 per cent, respectively, or a factor of 2.3.