The mammographic features of carcinoma originating within a fibroadenoma in 24 patients were studied by means of retrospective review of pathologic slides. Histologic examination showed that the lesions were lobular carcinoma in situ (LCIS) (seven patients), ductal carcinoma in situ (DCIS) (13 patients), synchronous LCIS and invasive lobular carcinoma (one patient), and synchronous LCIS and DCIS (three patients). In all patients the mammographic manifestation was a mass 1.0 cm or greater in diameter; 14 masses were 1-2 cm in diameter, and the remainder were more than 2 cm in diameter. Features that were considered suspect included large size, indistinct margins, and clustered microcalcifications. In three patients, microcalcifications within the mass raised suspicion of malignancy. At histologic examination these microcalcifications were associated with the intraductal carcinoma harbored in the fibroadenoma in only one of these patients. Fibroadenomas that harbor carcinoma may be indistinguishable from common benign fibroadenomas, but their occurrence is rare. In this study, a single patient had invasive lobular carcinoma; all the other lesions were in situ lesions.