Fibroadenoma of the breast is a benign neoplasia with a mixture of glandular and mesenchymal elements of high incidence and is mainly present in young women. This aspect and the presence of estrogen and progesterone receptors reinforce its hormone dependency. Recent studies have demonstrated that the proliferative activity of the mammary epithelium is significantly increased during the luteal phase. In order to assess whether fibroadenomas behave like the normal breast under the cyclic action of hormones we counted the number of mitoses and calculated the nuclear volume of epithelial cells in both the proliferative and luteal phases. We selected 20 women with clinically diagnosed fibroadenomas, 10 in the proliferative phase and 10 in the secretory phase. Our results showed no significant differences in mitotic index or nuclear volume between the proliferative and the luteal phases of fibroadenomas. We therefore conclude that the epithelium of fibroadenomas behaves like a true neoplasia whose regulation may occur via paracrine mechanisms, which in turn would explain its self-limited growth.