Abstract: The location of positive margins in lumpectomy specimens for ductal carcinoma could be predicted due to the common pattern of the geographic relationship between the intraductal and invasive carcinomas. To test this hypothesis, 62 lumpectomy specimens for ductal carcinoma of the breast were submitted for this study. The specimens were microscopically examined by serially sectioning them into giant sections in a plane parallel to the chest wall (frontal plane). The margins were identified as proximal (closest to the nipple), distal (opposite to proximal), and peripheral (nonproximal or distal). We found that the location of positive or close margins was proximal in 6 cases, peripheral in 13 cases, and none were found to be distal. Ductal carcinoma in situ (DCIS) was found to be located in the area adjacent to the invasive carcinoma. The invasive carcinoma was located at the periphery of the intraductal carcinoma. All six specimens with invasive carcinoma without DCIS had free margins. Nine of 16 specimens (56%) with extensive intraductal carcinoma (EIC) component and 7 of 40 (18%) with DCIS but negative EIC contained positive or close margins involved by DCIS. One case with multifocal invasive carcinoma measuring 3.5 cm in diameter and with DCIS but EIC negative had margins involved by both DCIS and invasive carcinoma. In conclusion, in ductal carcinoma, invasive carcinoma arose at the peripheral areas of the DCIS. DCIS tends to spread toward the nipple and the peripheral margins of the resected specimens. Incomplete excision of the ductal carcinoma and the wide positive margins are most likely caused by the failure to estimate the extent and location of DCIS.