For the purpose of studying the epidemiological situation of hyperthyroidism, the prevalence and incidence of the disease were studied in the study. During the four years from 1965 to 1968, the authors carried out a survey of hyperthyroidism combined with thyroid disease in unselected inhabitants in fifteen nongoitrous areas in Nagano Prefecture, Japan.All the inhabitants were examined for goiter. There were 73, 045 inhabitants in these areas and 59, 106 subjects (80.9 %) were examined. The total number of thyroid cases detected in the survey was 2, 098, i. e. 3.5 per cent of the examined had thyroid disease.Forty-seven cases (12 males and 35 females) of them were diagnosed as hyperthyroidism. The following results were observed regarding hyperthyroidism: 1) prevalence rate of the disease was 0.8 (0.4 for males and 1.1 for females) per 1, 000 subjects examined, 2) prevalence rate of the disease by sex and area was the same in different areas in both sexes except Area C, 3) prevalence rate of the disease by sex and age-group was not the same in different age-groups in females, though the same in males, 4) about one-third of the patients with the disease was unaware of the presence of it, in spite of dominant clinical signs and symptoms.Following the above mentioned survey, the incidence rate of hyperthyroidism, in one year and two years, was studied in four areas by studying the prevalence rate of thyroid diseases. Though the number of the cases examined in each programme was 6, 848 and 5, 222 respectively, only one case of hyperthyroidism was detected in each study. As a result, it was found to be impossible to determine the actual incidence rate of the disease per one year and per two years. In order to determine the incidence rate of the disease statistically, further examination of the inhabitants will be required.