The incidence of thyroid cancer has been reported to be 0.5-1.3%, when assessed by sonographic examination and 3.7-28.4% by histologic examination at autopsies. These incidences are much higher than those of clinically evident thyroid cancer, which are 2.0/100,000 for males and 7.2/100,000 for females, reported in Japan. In iodine deficient areas, chronic stimulation by TSH causes multinodular autonomous growth and function, leading to hyperthyroidism in middle-aged and elderly subjects. Incidence of Plummer's disease among Japanese with sufficient iodine intake is very low, accounting for 0.5-0.8% of all thyroid nodules and 0.3% of all thyrotoxic patients. The Plummer/Graves ratio was higher than 1 in endemic goiter area before iodine supplementation. Iodine intake affects the type of thyroid carcinoma. Decreased intake of iodine is associated with higher frequency of follicular and anaplastic cancers and lower frequency of papillary cancer. The high prevalence of papillary cancer(>85%) with good prognosis may explain the preferred selection of partial rather than total thyroidectomy in Japan.