Background: Positive lumpectomy margins are associated with increased rates of LR after BCT; however, the effect of increasing negative margin distance on LR is controversial. Rates of LR vary by breast cancer subtype, with the highest rates seen in TN cancers. The purpose of this study was to examine the rate of LR in relationship to margin width in TN breast cancers treated with BCT. Methods: All women with TN breast cancer who underwent BCT between 1999-2009 were identified from a prospectively maintained database. Margins were defined as positive (tumor on ink), close (≤2mm), or negative (>2mm) for either invasive or in situ cancer. Patients with positive margins (n=46) were excluded. Statistical comparisons were by t tests, Fisher’s exact test, and Wilcoxon rank sum test. Cumulative incidence of LR was compared using competing-risks methodology. Results: Characteristics of the 535 cancers (in 534 patients) with close (n=71) and negative (n=464) margins are compared in the Table. At median follow-up of 84 months (range, 8-165 months) there were 37 local, 18 regional, and 77 distant recurrences or deaths as first events. 10 patients had LR prior to planned radiation therapy (RT). The cumulative incidence of LR at 60 months for patients with close margins was 7.3% (95% CI, 1.1-13.6), and 5.1% (3.0-7.2) for negative margins. After controlling for use of chemotherapy, there was no significant difference in LR between the entire close and negative margin groups (p=.07) or after exclusion of recurrences prior to RT (p=.06). A difference in the risk of distant recurrence or death was not observed (p=.60). Conclusions: Margin width greater than 2mm was not associated with a clinically meaningful reduction in LR risk. This, coupled with reports of similar LR in TN cancers treated with BCT or mastectomy, suggests that larger surgical procedures do not improve outcomes in this high-risk patient subset.