We investigated the expression of the retinoblastoma gene product (pRB) in various clinical thyroid tissues. Follicular cells in the normal thyroid sometimes expressed pRB but their positive cell ratio was always less than 10% as it was in hyperplasia. All of the 57 carcinomas tested expressed pRB immunohistochemically with various positivity and 36 of them (63.2%) showed high immunoreactivity. In contrast, only four of the 15 follicular adenoma samples (26.7%) revealed moderate pRB immunoreactivity and there was a statistically significant difference (p<0.01) in PRE positivity between the carcinomas, including follicular ones, and benign follicular adenomas. These findings indicate that pRB may be closely related to the suppression of tumoral growth, especially of carcinoma growth of the thyroid. Western blot analysis showed pRB immunoreactivity for all tested samples to be a single, sharp band located at 110 kDa, indicating that pRB in thyroid tumors is always unphosphorylated and active. The present study suggests that frequent pRB expression in an active form could be one of the reasons for the slow growth and excellent prognosis of thyroid carcinoma.