The extent of excision prior to radiation for breast cancer is controversial. Three hundred evaluable patients with invasive ductal carcinoma received radiation therapy after gross tumor excision. The median follow-up was 70 months. Local recurrence was related to the presence of an extensive intraductal component (EIC) in addition to the invasive ductal carcinoma. From operative notes and pathology reports, patients with an EIC were categorized as having discrete masses with the extent of disease confirmed histologically or as having one of five criteria in which the final pathologic findings revealed more intraductal disease than was evident grossly or by frozen section. After ten years of follow-up, the local failure rate of patients without an EIC (193) was 3% compared with 35% for those with an EIC (107). Eight-year recurrence rates were 18% for clearly defined tumors and 71% for tumors in which the intraductal component was detectable only histologically.