BACKGROUND Ductal carcinoma in situ (DCIS) is detected most commonly on routine screening mammography in the asymptomatic patient, and has a long natural history. The objective of the current study was to determine the long‐term outcome after breast‐conservation surgery followed by definitive breast irradiation for women with mammographically detected DCIS of the breast. METHODS In total, 1003 women with unilateral, mammographically detected DCIS of the breast underwent breast‐conserving surgery followed by definitive breast irradiation. These women were treated in 10 institutions in North America and Europe. The median follow‐up was 8.5 years (mean, 9.0 years; range, 0.2–24.6 years). RESULTS The 15‐year overall survival rate was 89%, and the 15‐year cause‐specific survival rate was 98%. The 15‐year rate of freedom from distant metastases was 97%. In total, there were 100 local failures (10%) in the treated breast. The 15‐year rate of any local failure was 19%, and the 15‐year rate of local only first failure was 16%. Patient age ≥ 50 years at the time of treatment and negative final pathology margins from the primary tumor excision both were associated independently with a lower risk of local failure in univariate analysis (P = 0.00062 and P = 0.024, respectively) and in multivariate analysis (P = 0.00057 and P = 0.0026, respectively). For favorable subgroups of patients age ≥ 50 years or with negative resection margins, the 10‐year risk of local failure was ≤ 8%. CONCLUSIONS The current results support the use of breast‐conserving surgery followed by definitive breast irradiation for the treatment of patients with mammographically detected DCIS of the breast. Patient age ≥ 50 years at the time of treatment and negative resection margins both were associated independently with a decreased risk of local failure. Cancer 2005. © 2005 American Cancer Society. The outcome after breast‐conservation treatment with radiation is presented for 1003 women who had mammographically detected ductal carcinoma in situ of the breast. Patient age ≥ 50 years and negative final pathology margins both were associated independently with a lower risk of local failure.