Smooth muscle cells are extremely rare stromal components of fibroadenoma. Eighty-five cases of fibroadenoma were reviewed in order to investigate the frequency of smooth muscle cells and to discuss its origin. Of the 85 cases, four (4.7%) cases showed smooth muscle cells in the stroma. Distribution in terms of age and tumor size did not show any difference from other cases. Three cases were classified as an intracanalicular subtype, and one was a mastopathic subtype. The cells possessed most of the ultrastructural characteristics of smooth muscle cells, but poor indentation of nuclear contours suggested the last remnant of fibroblasts. It was therefore easy to recognize the cells derived from fibroblasts. In two of four cases, smooth muscle cells were observed in the stroma, which had marked hyalinization and calcification. Therefore, it was supposed that smooth muscle cells could appear in the stroma of long-standing tumors.