In a review and reclassification of 5,560 benign epithelial lesions of the breast entered in the files of the Laboratory of Surgical Pathology at Columbia, we found 211 examples of the type of lobular proliferation occurring alone without co-existing infiltrating carcinoma, which we prefer to call lobular neoplasia, but which is generally referred to as noninfiltrating lobular carcinoma in situ. We regard this lesion as a separate distinctive pathological-clinical entity. These 211 cases are studied from a number of parameters, including the ages of the patients, the breast affected, the length of the follow-up, the interval between the initial diagnosis and the frank carcinoma which eventually developed in 17.1 percent of the patients. The relationship of microscopic qualitative and quantitative variations in the lobular neoplasia to subsequent carcinoma was studied; the variations were not found to have any value in predicting subsequent carcinoma. This study is unique in that we have data as to the frequency of a family history of carcinoma in a mother or sister, and also as to the occurrence of gross cystic disease in our patients with lobular neoplasia. We have determined the ratio between the observed and expected numbers of patients developing carcinoma in the several possible combinations of these three factors which predispose to carcinoma. We report that the predisposition is cumulative: in patients in whom all three predisposing factors were present the ratio of observed to expected risk of carcinoma was 13:8. We do not recommend mastectomy for lobular neoplasia, but only systematic follow-up by palpation of the patients' breasts every four months.