From June 1985 to August 1989, 344 cases of mammographic non-palpable breast lesions were operated on at the National Cancer Institute in Milan. The mammographic findings consisted of clusters of microcalcifications in 162 cases (42.1%), suspicious opacities with irregular borders in 116 cases (37.7%) and opacities containing microcalcifications in 66 cases (19.2%). The mean age of the patients was 51 years (range 31-77 years). In all patients localization was performed 1 day before the operation, introducing a self-retaining anchor wire into the mammary parenchyma. The histological findings showed benign breast disease in 184 cases (53.4%); proliferative dysplasia without atypia in 150 cases (81.5%); proliferative dysplasia with atypia in 22 cases (12%); fibroadenoma in nine and papilloma in three cases. Of 160 patients with carcinoma, 37 had non-infiltrating carcinoma: 28 of these cases (17.5%) had non-infiltrating ductal carcinoma and nine cases (5.6%) had lobular carcinoma in situ. In the 123 cases with infiltrating breast cancer the histological types were ductal infiltrating carcinoma (32.5%), lobular infiltrating carcinoma (9.8%), and 34.1% of the cases an associated or prevalent intraductal carcinoma was found. In 138 cases (85.6%) a conservative surgical procedure (quadrantectomy or more limited excision) was done, and in 22 (14.4%) cases a total mastectomy was necessary because of the extent of the disease. Axillary dissection was performed in 116 of the 123 patients with histological invasive carcinoma. Nodal metastases were found in 24 cases (20.7%), and only one node was involved in nine of the cases (37.5%), two to three nodes in nine cases (37.5%) and four to ten nodes in six cases (25%).