Background: The indications for postmastectomy radiotherapy (PMRT) are expanding to include patients 1-3 axillary nodal metastases (ALN). Improvements in diagnostic evaluation have led to increasing numbers of breast cancer (BC) patients who are found to have microscopic nodal metastases (N1mic). The challenge today is whether these BC patients have risk that warrants the routine delivery of PMRT. Methods: We reviewed patients with pathologic T1-2N1 BC treated with initial mastectomy (mast) and adjuvant systemic therapy (ST) from 2000-2013. The primary endpoint was locoregional failure (LRF), defined as a recurrence in either the ipsilateral chestwall or regional lymphatics (axillary, internal mammary, or supraclavicular). Secondary endpoints were disease-free survival (DFS, failure or death) and overall survival (OS). The log-rank test was used to compare survival between groups. Results: We identified 550 eligible patients from our prospectively maintained cancer registry with 5 year median follow-up. 95 patients (17%) had N1mic disease. Baseline characteristics include: median age 53 yrs, 61% pathologic T2, 39% grade 3, 72% hormone receptor positive, 16% HER2+, 12% triple-negative. Treatment included chemotherapy in 78% (n = 428), PMRT in 15% (n = 82), and anti-endocrine therapy in 70% (n = 385). A median of 18 ALN (range, 1-68) were removed. Among the patients with N1mic disease, 81 had 1+ node, 13 had 2+ nodes, and 1 had 3+ nodes. The 5 yr LRF was 0% for patients with N1mic disease vs. 4.6% in those macro metastases (p = 0.84). The 5 yr LRF rate for the entire cohort was 3.9%; patients with 1+, 2+, and 3+ nodes had 5 yr LRF of 2.6%, 4.7% and 6.4%, respectively (p = 0.79). Patients with N1mic disease had a trend towards improved DFS (91.6% vs. 82.3%, p = 0.07) and significantly improved OS (96.9% vs. 87.6%, p = 0.03) compared to patients with macrometastases. Conclusions: In a cohort of patients with T1-2,N1 BC treated with modern therapy, we found overall low rates of LRF. Patients with N1mic disease had no LRF events and improved OS compared to patients with macrometastases. These findings support that PMRT should not be routinely recommended for N1mic BC patients with T1-2 tumors.