Purpose: Indications for postmastectomy radiation include primary tumor size > 5 cm and ≥ 4 positive axillary nodes. In clinical practice, patients with a close or positive margin after mastectomy are also often treated with postmastectomy radiation. However, there is little data regarding the risk of a chest wall recurrence in patients with this pathologic feature who otherwise would be considered low risk (tumor size < 5 cm and/or 0-3 positive nodes). To address this issue, we assessed the risk of a chest wall recurrence in women with stage I-II breast cancer who underwent mastectomy and were found to have primary tumor size < 5 cm and 0-3 positive nodes with a close or positive deep margin.Methods and Materials: Between 1985 and 1994, 789 patients underwent mastectomy and 16% had tumor within 1 cm of the deep resection margin. The study population consists of 35 of these patients whose primary tumor size was < 5 cm with 0-3 positive axillary nodes and who received no postoperative radiation. The median age was 44 years (range 29-76). 44% had T1 tumors and 56% T2 tumors. Pathologic axillary nodal status was negative in 65% and positive in 35%. The median number of positive nodes was 1. The deep margin was positive in 2 patients, ≤2 mm in 17 patients, 2.1-4 mm in 7 patients and 4.1-6 in 9 patients. 66% received adjuvant chemotherapy (12 CMF, 10 CAF, 1 CA) ± Tamoxifen and 20% received Tamoxifen alone. The median follow-up was 5 years (range 0.6 - 11.9).Results: There were 4 chest wall recurrences at a median interval of 2 years (range 0.6-7.2). Two were associated with distant metastases and one with an axillary recurrence. The 5 and 8 year cumulative incidence of a chest wall recurrence were 9% and 18%. Patient age correlated with a cumulative incidence of chest wall recurrence > 25% at 8 years: Age ≤ 50 years was 28% vs. 0% for age > 50 (p=0.08). There was no correlation with chest wall failure and number of nodes, ER status, lymphovascular invasion, location of primary, grade, family history or type of tumor close to the margin. All four chest wall failures were in patients who had received adjuvant systemic chemotherapy ± Tamoxifen. Chest wall failures occurred in patients with margins within 1-2 mm in three patients and 5 mm in one patient. The cumulative chest wall recurrence at 8 years by margin proximity was 24% ≤ 2mm vs. 7% 2.1-6 mm (p=0.44), and by clinical size 24% for T2 tumors vs. 7% for T1 (p=0.55).Conclusions: A close or positive margin is uncommon (< 5%) after mastectomy in patients with tumor size < 5 cm and 0-3 positive axillary nodes, but when present it appears to be in a younger patient population. The subgroup of patients aged 50 or younger with clinical T1-T2 tumor size and 0-3 positive nodes who have a close (≤ 5 mm) or positive mastectomy margin are at high risk (28% at 8 years) for chest wall recurrence regardless of adjuvant systemic therapy, and therefore we recommend they be considered for postmastectomy radiation. This high risk of local failure was not observed in patients over age 50, suggesting this subgroup may not require adjuvant chest wall irradiation.