Lobular Neoplasia of the breast, a term which we prefer to that of Lobular Carcinoma in Situ, is a multifocal microscopic entity of uncertain and controversial clinical significance. Lobular Neoplasia is a incidental histological finding among otherwise benign breast biopsies (0.5%-3.5%) depending mainly upon the diagnostic intensity. Its extent is rarely so massive that it makes up a palpable tumor. Although this lesion may be demonstrated at all ages after puberty it occurs most often in premenopausal women. The multicentricity is not below 70% and bilateral occurrence not below 30-35%. Demonstration of Lobular Carcinoma in situ means a 7-12 times greater risk of later developing invasive breast carcinoma as compared with an age corrected general population. Both breast are at risk and about one half of these subsequent carcinomas will develop in the controlateral breast. Attempts at correlating the histological appearances in Lobular Neoplasia with the subsequent occurrence of invasive breast carcinoma have generally be negative. From this data it appears that ipsilateral mastectomy will protect only one half of women with lobular neoplasia who will eventually develop carcinoma and the only surgical approach would perform a bilateral mastectomy. As an alternative to mastectomy, a careful program of frequent follow-up examinations, when possible is preferable.