Postmastectomy irradiation substantially reduces the risk of locoregional recurrences (LRR) of breast carcinoma. However, debates continue on the prognostic factors for radiotherapy and the effect of radiotherapy on overall survival. The present study was undertaken to investigate the prognostic significance of level III positive nodes, along with the other factors for LRR, and to evaluate the effect of postmastectomy radiotherapy on overall survival among node-positive breast carcinoma treated systemically. Data from 549 consecutive node-positive breast cancer patients who underwent modified radical mastectomy and received adjuvant systemic therapy were studied retrospectively. Prognostic factors for LRR and the effect of postmastectomy radiotherapy on overall survival were analyzed. Survival curves were generated by the Kaplan-Meier method, and multivariate analysis was performed by the Cox proportional hazard model. The 5-year locoregional recurrence rate is 7%. Apical invasion was found to be an independent prognostic factor for LRR (HR 2.6, CI 1.29-5.35) along with a finding of 4 or more positive nodes and T3 tumor. Adjuvant radiotherapy decreased LRR and improved survival significantly. Apical invasion, 4 or more positive axillary lymph nodes, and T3 tumor are the predictors of LRR, and patients with these adverse factors are candidates for adjuvant radiotherapy. Postmastectomy radiotherapy improves overall survival.