Local tumor extension was studied using a continuous series of multiple blocks of mastectomy specimens to assess malignancy remaining after breast-conserving surgery for early-stage breast cancer. In this study, 183 cases were chosen, consisting of 6 noninvasive ductal carcinoma cases and 177 invasive ductal carcinoma cases. The histopathology in 59 (32%) of the 183 cases corresponded to that showing extensions of more than 2.6 cm from the tumor margin. These wide extensions were also seen in 17% of breast cancers with a tumor size of less than 2 cm. The incidence of wide extension was higher in younger patients with cases of noninvasive ductal carcinoma. Extension to the nipple-areola was seen in 14% of cases with a tumor size of less than 2 cm. Breast cancers with multicentric development accounted for 3% of those with a tumor size under 2 cm. These findings suggest that if lumpectomy is performed with a margin of 2 cm for tumors with a size of 2 cm or less, a cancerous residue would be found in the surgical margin of 15–20% of the cases. The actual incidence was 23% of cases after breast-conserving treatment in our study. On the basis of the data, breast-conserving treatment with only local resection of the primary lesion showed cancerous residue such as intraductal cancerous extension in about 20% of cases. Therefore, it was concluded that, as part of breast-conserving therapy of early-stage breast cancer, radiation therapy of the whole breast should be performed after surgery with clear margins to Control local recurrence. © 1993 Wiley-Liss, Inc.