Previous studies have shown that C cells are twice as numerous in male than in female thyroids and that C cell hyperplasia (CCH) is much more frequent in men. These findings suggest regulation involving sex steroid hormones through the expression of sex steroid hormone receptors on C cells. To investigate this hypothesis, we performed an immunohistochemical study of estrogen receptors alpha (ER alpha) and beta (ER beta), progesterone receptors (PR), and androgen receptors (AR) on specimens from a series of 40 patients operated on for a medullary thyroid carcinoma (MTC; n=28; female 18, male 10) and/or CCH (n=19; female 6, male 13). ER beta was the only receptor to be consistently expressed in CCH (100%) and MTC (96.5%), whereas ER alpha was never expressed. PR and AR were rarely expressed in MTC (7 and 14%, respectively). AR was expressed in half the CCH cases (53%), with a trend to male predominance (61% in men vs 33% in women). Our study is the first to describe ER beta expression in CCH. In addition, our findings suggest that CCH, and possibly MTC, might be influenced by sex steroid hormones, namely, estrogens and androgens, through the expression of ER beta and AR on C cells.