559 Background: Recent national consensus guidelines regarding optimal margin width for the management of DCIS have been published; however, controversy remains for managing margins <2mm. The relationship between margin width and locoregional recurrence (LRR) was determined in a contemporary cohort of patients. Methods: 1504 patients with DCIS undergoing definitive breast conserving surgery from 1996 to 2010 were analyzed for clinical and pathologic characteristics from a prospectively managed comprehensive academic cancer center database. Cox proportional hazard models were used to examine the relationship between margin width (<2mm or ≥2mm) and LRR by adjuvant radiation therapy (RT). Patients with positive margins (n=11) were excluded. Results: Overall, 3.4% of patients had a LRR at a median follow-up of 8.7 years. Univariate analysis of age, family history, grade, tumor size, comedonecrosis, RT, adjuvant hormonal therapy, ER status, and margin width found younger age (< 40 yr, p=0.02), no RT (n=299, p=0.005), and margin width <2mm(n=138, p=0.005) to be associated with LRR. The association between margin width and LRR differed by adjuvant radiation therapy status (p=0.02 for the interaction). There was no statistical significant difference in LRR for patients with margins <2mm vs ≥2 mm who received RT, (10-year LRR 6.0% vs 3.2%, respectively; HR 1.5, 95% CI 0.5-4.2, p=0.48). For patients who did not receive RT (n=299), those with margins < 2 mm were significantly more likely to develop a LRR than those with margins ≥2mm (10-year LRR 35.7% vs. 4.6%, respectively; HR 7.2, 95%CI 2.6-19.4, p=0.0001). Conclusions: In patients with <2mm margins receiving adjuvant radiation therapy, there is no difference in locoregional recurrence when compared to patients with ≥2mm margins. Additional surgery for wider margins of resection are not routinely justified in this group of patients but should be obtained for patients with <2mm margins who forego radiotherapy.