In order to assess the utility of margin width in relation to other histopathologic features as a determinant of local control in ductal carcinoma in situ (DCIS) of the breast, we retrospectively examined the treatment of 109 breasts treated with (n โซุโฌ 54) or without adjuvant radiotherapy (n โซุโฌ 55). Median follow-up was 49 and 54 months for patients treated with excision alone (E) or excision plus adjuvant radiotherapy (EุXRT), respectively. Cases treated with EุXRT were significantly larger and had a trend towards closer surgical margins than those treated with E alone. For all cases, margin width <1 mm and lesion diameter >15 mm were significantly associated with increased local recurrence. Lesion size <15 mm was associated with no cases of local failure regardless of treatment arm. For lesions >15 mm in diameter, there was a significant decrease in 5-year local failure with EุXRT compared to E alone (21% vs. 36%, P โซุโฌ 0.03). Tumor margin >1 mm was associated with a low rate of 5-year local failure for either E alone or EุXRT (10.9% vs. 4.6%, P โซุโฌ NS). Tumor margin <1 mm had a high rate of local failure that was not significantly decreased by the addition of adjuvant radiotherapy. These results show that large diameter (>15mm) and close surgical margins (<1 mm) are the dominant risk factors for local recurrence in DCIS. EุXRT significantly decreased local failure risk compared to E alone for large lesions but not for those with close margins.