THE CONCEPT of carcinoma in situ, as opposed to carcinoma with invasion, is widely recognized. In general, the accepted treatment for in situ carcinoma is considerably more conservative than therapy for invasive cancer. The results of early treatment are usually good, and the consequences of neglect are clear when the natural history of the tumor has been fully described. The natural history of in situ lobular carcinoma of the breast is not completely delineated. Since the original description by Foote and Stewart1 in 1941, fewer than 40 cases have been reported, making therapeutic decisions for women with this disease difficult. Our purpose is to report 13 patients with in situ lobular carcinoma of the breast treated at the University of Chicago, nine of whom have been followed for five years or more, with the longest follow-up being 24 years. These patients are presented particularly because of the duration of