Two hundred cases of breast carcinoma were studied, with particular attention to adjacent non-malignant modifications considered by some authors as preneoplastic changes. Diverse proliferative lesions were recognized in 82% of cases. Atypical hyperplasia was present in 51.5% of cases and was more often ductal (83/200) than lobular (40/200). Microscopic cysts were noted in 72% of cases. Calcifications, easily identified by mammography were present in 20% of cases and were associated with atypical hyperplasia in 4/5 of cases. The mean of patients was 58 years. For comparative studies of histologic lesions the following age categories were evaluated: category I < 50 years (57 patients), category II .gtoreq. 50 years (143 patients). Atypical lobular hyperplasia, adenosis, and, to a lesser degree, sclerosing adenosis and macroscopic cysts were more frequently associated with breast cancer in women younger than 50. Fibro-atrophic or lipomatous changes were more often found in the older group of women, as expected. Sclerosing adenosis was significantly more frequent in breasts with multifocal infiltrating carcinoma. Such changes must be considered as risk factors by pathologists when present in mammary specimens without carcinomatous lesions, and careful study should be undertaken to locate a possible in situ microscopic carcinoma. No differences appeared in lesions associated with breast cancer in women with or without a familial history of breast cancer.