Ninety-eight women with lobular carcinoma in situ (LICS) of the breast were identified over a 16-year period. Consecutive slide review of all breast material over a 12-year period identified 25 women with LCIS on biopsy who did not undergo mastectomy. Only 1 woman (4%) in a complete followup averaging 17.5 years developed ipsilateral invasive carcinoma. of 32 women with a contralateral breast at risk, 3 (9.7%) developed infiltrating carcinoma. LCIS was found with infiltrating carcinoma, especially of the lobular (small cell) type, with such frequency as to indicate a close relationship. However, the risk of subsequent development of infiltrating carcinoma in the breast with biopsyproven LCIS is shown to be substantially less than indicated by previous authors. This suggests that careful and prolonged followup may suffice for the woman whose breast biopsy contains lobular carcinoma in situ.