Gross excision of the tumor followed by radiotherapy is used for treatment of early breast cancer. Local recurrence after this form of treatment is uncommon except in patients with infiltrating ductal carcinoma whose excision specimens reveal an extensive intraductal component (EIC). It is unclear whether this observation is due to a qualitative difference in the response of tumors with EIC to radiation or to a quantitative difference in the tumor burden remaining after gross excision in patients with EIC. To address this question, the authors examined pathologic material in 71 patients with infiltrating ductal carcinoma treated with gross excision of the tumor and then selected for re-excision of the tumor site prior to radiotherapy because of the presence of either EIC in the primary excision specimen or microscopic tumor at or close to margins in the initial excision. Residual carcinoma was seen in 62% of all patients but was more frequent among the 25 patients with EIC than the 46 without (88% vs. 48%, P = 0.002). The nature of the residual tumor differed for patients with and without EIC in the primary excision specimen. Residual carcinoma in patients with EIC was often widespread and composed predominantly of intraductal carcinoma. In contrast, residual tumor in patients without EIC usually consisted of only scattered microscopic foci of infiltrating and/or intraductal carcinoma. The authors conclude that patients with EIC treated with gross excision of the tumor frequently have considerable residual intraductal carcinoma near the primary site. This finding may account for the increased risk of local recurrence observed in patients with EIC treated without re-excision of the tumor site before radiation therapy.