Elastosis in the breast is an unusual phenomenon and its morphogenesis has not yet been fully ascertained. The degree of elastosis in the breast associated with benign diseases, including fibroadenoma and fibrocystic disease as well as with breast carcinoma, was examined with special reference to the correlation between the degree of epithelial proliferation and elastosis. Using the immunohistochemical method, the presence of elastase (EL) and α 1-antichymotrypsin (ACT), one of the protease inhibitors, in these epithelial cells was also investigated to elucidate the role of an imbalance in these enzymes in the morphogenesis of the elastosis. Consequently, it was shown that there is a tendency in fibroadenoma and fibrocystic disease, for epithelial proliferation to be related to the degree of elastosis, and that the lack of EL in proliferated epithelial cells might play a role in the occurrence of elastosis, although ACT has no significant correlation. On the contrary, in our study noninvasive carcinoma showed marked periductal elastosis but no stromal elastosis, while invasive carcinoma showed various degrees of periductal and stromal elastosis. In invasive carcinoma, especially scirrhous carcinoma, the degree of ACT in cancer cells correlated well with stromal elastosis, although there was no correlation with EL. These findings suggest that an imbalance of the protease-antiprotease system, produced by epithelial cells of the breast, contribute to the morphogenesis of elastosis, although the physiological event, aging, is only marginally related to elastosis. Further investigation of the cells producing elastin and regulatory factors may be necessary.