Lobular carcinoma may arise within the epithelial component of fibroadenoma of the breast, as evidenced by 5 cases reported from the files of the Mayo Clinic and 21 cases cited in the literature. The 5 cases reported here occurred in a series of 4000 cases of fibroadenoma during a 43-year period. Lobular carcinoma is the more frequent type to be seen in fibroadenoma (in 22 of 26 cases); it usually develops in situ. Extra-adenomatous carcinoma of the ipsilateral breast was identified in 11 of 26 cases, and 3 cases of contralateral carcinoma were noted in the entire series. Thus, lobular carcinoma arising within the epithelial component of a fibroadenoma has biological features similar to the behavior of lobular carcinoma of the breast in general. The prognosis has been favorable; lesions are usually encountered early; in only 2 of 26 cases were there axillary metastases.