One hundred twenty-nine biopsies from 121 patients with a frozen or paraffin section diagnosis of noninvasive breast carcinoma were studied. Eight women had bilateral noninvasive carcinoma. Seven biopsies reported as intraductal on frozen section contained invasive carcinoma on paraffin section. Of the remaining 122 biopsies proven to have noninvasive carcinoma on paraffin section, 39 (34%) were reported at frozen section and as noninvasive carcinoma, 24 (20%) as atypical and 59 (48%) as benign. Intraductal carcinoma (IDC) was identified more often at frozen section (45%) than was lobular carcinoma in situ (19%). Among 41 patients who had bilateral carcinoma with invasive disease in one breast, 76% of contralateral noninvasive carcinoma was LCIS. After excisional biopsy, carcinoma was found in 56% of 103 mastectomy specimens, including invasive carcinoma in 6% of breasts with IDC and 4% with LCIS. Residual noninvasive carcinoma was usually of the same type found at biopsy (90% IDC and 88% LCIS) and involved quadrants other than the biopsy site in 33% with IDC and in 80% with LCIS. When the frozen or paraffin section diagnosis of a generous excisional biopsy was noninvasive breast carcinoma, there was a substantial risk that foci of the same type of noninvasive carcinoma were also present in other quadrants. However, occult foci of invasive carcinoma were quite infrequent and the risk of axillary metastases was very low. Adequate treatment for noninvasive carcinoma requires elimination of all residual foci of noninvasive disease. At present this can best be accomplished by total mastectomy if the operation is properly performed. To insure removal of the axillary extension of the breast and for staging, in continuity dissection of the lowest axillary lymph nodes is also prudent.