Improvements in mammography in the past 25 years have made it possible to detect before surgery many lesions with a high probability of being pre-invasive carcinoma. Because these cancers are virtually all cured by mastectomy, there has been considerable interest in alternative types of treatment. Retrospective studies of pre-invasive carcinoma treated by biopsy only revealed subsequent carcinoma in 30 to 40% of patients. Among women with lobular carcinoma in situ (LCIS), the frequency of subsequent carcinoma was nine times the expected rate, and mortality due to the disease was 11 times greater than expected. The risk of later invasive carcinoma appeared to involve both breasts equally when LCIS was present and to be largely limited to the breast that harbored intraductal carcinoma (IDC). When mastectomy was performed for pre-invasive carcinoma, unsuspected invasion was found in 4% of patients with LCIS and 6% with IDC.It remains to be determined whether multicentric preinvasive carcinoma will follow the same course in patients with palpable invasive carcinoma treated by partial mastectomy and some form of adjuvant therapy. At present, it is prudent to treat pre-invasive breast cancer by ipsilateral mastectomy with low axillary dissection and to perform a contralateral breast biopsy. However, prospective, controlled investigations are urgently needed to identify groups of patients with a high or low risk of developing invasive carcinoma and to determine whether non-surgical treatment can alter the course of pre-invasive disease. By pursuing these studies, it may ultimately be possible to measure the success of breast cancer detection by the number of patients cured without mastectomy. Accepted for publication November 7, 1979.nomas, there was an appreciable risk of metastases. Metastases were documented in axillary lymph nodes of 13.4% of patients who had invasive cancer and who had at least 5 lymph nodes examined. Axillary metastases were found in 20.2% of cases with primary tumors 0.5-0.9 cm in diameter reported by the SEER program of the National Cancer Institutez8 and in 23.1%of a series of patients with clinically occult or nonpalpable invasive carcinomas detected by mammography in a recent clinical series.z6 Few have questioned the clinical significance of the small invasive tumors, despite debate over therapy, and therefore, the detection of these lesions was generally considered to be a beneficial result of breast cancer screening, including mammography. The advantage gained by detecting pre-invasive or in situ carcinoma has been more controversial. 134329 Variation in the frequency of the lesions depends in part on the clinical setting and may influence the intensity of debate about treatment. Intraductal carcinoma and LCIS each constitute about 5% of all carcinomas diagnosed in our institution.lR However, among non-palpable lesions detected by mammography 78% were pre-invasive, and of these, nearly 90% were intraductal. In the setting of screening mammography, the recent BCDDP review revealed that ...