Detection of lobular in situ mammary carcinoma (LIS) requires microscopic study of the breast. To improve the yield of necessarily blind biopsies, we plotted the spatial distribution of LIS in nine breasts, eight of which were totally and one partially embedded. Lobular in situ carcinoma is concentrated in the upper quadrants beneath the areolar periphery; 47.8% is found within 5 cm of the nipple. Peripherally, LIS involves radial mammary sectors, often skipping contiguous radii. To detect LIS, all breasts requiring surgery should be entered through a circumareolar incision. Biopsies should be taken routinely of breast tissue lying 3 to 4 cm from the nipple in an arc perpendicular to radii from the nipple, sampling all upper-quadrant duct systems. With proper tissue processing in pathology laboratories, such biopsies should detect LIS, if present, with more than 90% success.