It has been our impression that in-situ lobular carcinoma is diagnosed less frequently than the true incidence of the disease. To confirm this, all breast biopsies at George Washington University Hospital, from 1953 to 1971, were reviewed and the incidence of in-situ lobular carcinoma was determined to be 2.5 per/100 breast biopsies. Four hospitals were contacted and asked to review their records to ascertain the incidence of the disease at these institutions. Two hospitals reported never diagnosing the lesion and two hospitals diagnosed it at a much lower rate. Reasons for the difference are discussed. One hundred twenty-four cases of in-situ lobular carcinoma were diagnosed, and 105 cases received a simple mastectomy. Nineteen patients refused operation and instead were followed carefully. The good results in the latter group and reports by other authors suggest to us that a program of careful follow-up visits is a valid alternative to simple mastectomy in selected patients with in-situ lobular carcinoma.