Three hundred and thirteen cases of human thyroid tissues, comprising 39 nodular goiters from 34 females and 5 males, 130 adenomas from 93 females and 37 males, and 144 carcinomas from 99 females and 45 males were used for the present immunohistochemical assessment of estrogen receptor (ER) expression. Thirty-three cases of follicular carcinoma, 115 cases of papillary carcinoma and 6 cases of anaplastic carcinoma were included in the malignant tumor group. Incidences of ER-positive cases were 23/39 (58.9%) for nodular goiter, 44/130 (33.8%) for adenoma and 26/144 (18.0%) for cancer. In the individual carcinoma categories, 7/23 (30.4%) follicular, 19/115 (16.5%) papillary and 0/6 (0%) anaplastic lesions were judged as positive cases. Thus, the incidence of ER-positive cases tended to decrease with the degree of malignancy; this trend being similar in both sexes. Moreover, the average ages of ER-positive cases were lower than those of ER-negative cases for all types of thyroid carcinoma except the follicular variety in males. It was thus suggested that ER expression may be related to prognosis and tumor growth at early stage. Since the incidence of ER does not significantly differ between females and males, the observed sex differences regarding thyroid tumor incidence may reflect the higher estrogen serum content in females.