A series of 87 cases of carcinoma in situ of the breast was reviewed. In 49 cases in which lobular carcinoma in situ was shown on biopsy, three patients were found to have inyasive carcinoma in the subsequently done mastectomy. All three of these cases were in a group of 14 patients shown to have bilateral lobular carcinoma in situ on biopsy. In a group of 38 patients with intraductal carcinoma on biopsy, seven were found to have invasive carcinoma in the mastectomy that was subsequently done. Lymph node metastases were found in one patient in the lobular group and four patients in the intraductal group. Three patients in the intraductal group died of cancer. None in the lobular group has died of cancer.