Three benign mammary disorders which are infiltrative and which mammographically and clinically may simulate malignancy are: sclerosing adenosis; fat necrosis; and plasma-cell mastitis. Each of these disorders incites a desmoplastic reaction which intensifies the cellular infiltration that is already present. The proliferating fibrous tissue distorts the cellular components of the lesions and results in extensive infiltration by these cells of the surrounding breast tissue. Whereas duct carcinomas tend to appear localized and homogeneous in mammograms, benign infiltrative lesions, as reflected in their pathology, tend to appear ill-defined and mottled. The dictum that radical mastectomy should never be performed for a clinically and roentgenographically apparent carcinoma without preliminary histologic proof of the diagnosis obviously still holds true. Duct carcinomas exhibit a high degree of correlation between their mammographic and pathological infiltrative characteristics. The extent of local macroscopic infiltration as recorded by mammography may be of value as a guide to the frequency of concomitant axillary lymph node metastasis.