Importance: Evolving data on the effectiveness of postmastectomy radiation therapy (PMRT) have led to changes in National Comprehensive Cancer Network (NCCN) recommendations, counseling clinicians to "strongly consider" PMRT for patients with breast cancer with tumors 5 cmor smaller and 1 to 3 positive nodes; however, anticipation of PMRT may lead to delay or omission of reconstruction, which can have cosmetic, quality-of-life, and complication implications for patients. Objective: To determine whether revised guidelines have increased PMRT and affected receipt of breast reconstruction.We hypothesized that (1) PMRT rates would increase for women affected by the revised guidelines while remaining stable in other cohorts and (2) receipt of breast reconstruction would decrease in these women while increasing in other groups. Design, Setting, and Participants: Retrospective, population-based cohort study of Surveillance, Epidemiology, and End Results (SEER) data on women with stage I to III breast cancer undergoing mastectomy from 2000 through 2011. Our analytic sample (N = 62 442) was divided into cohorts on the basis of current NCCN radiotherapy recommendations: "radiotherapy recommended" (tumors >5 cm or≥4 positive lymph nodes), "strongly consider radiotherapy" (tumor≥5 cm, 1-3 positive nodes), and "radiotherapy not recommended" (tumors≥5 cm, no positive nodes). Main Outcomes and Measures: We used Joinpoint regression analysis to evaluate temporal trends in receipt of PMRT and breast reconstruction. Results: The 3 cohorts comprised 15 999 in the "radiotherapy recommended" group, 15 006 in the "strongly consider radiotherapy" group, and 31 837 in the "radiotherapy not recommended" group. Rates of PMRT were unchanged in the radiotherapy recommended (29.9%) and radiotherapy not recommended (7.4%) cohorts over the study period. Receipt of PMRT for the strongly consider radiotherapy cohort was unchanged at 26.9%until 2007. At that time, a significant change in the APC was observed (P=.01) with an increase in APC from 2.1% to 9.0% (P = .02) through the end of the study period, for a final rate of 40.5%. Breast reconstruction increased across all cohorts. Despite increasing receipt of PMRT, the strongly consider radiotherapy cohort maintained a consistent increase in reconstruction (annual percentage change, 7.4%) throughout the study period. This is similar to the increase in reconstruction observed for the radiotherapy recommended (10.7%) and radiotherapy not recommended (8.4%) cohorts. Conclusions and Relevance: Changes in NCCN guidelines have been associated with an increase in PMRT among patients with tumors 5 cmor smaller and 1 to 3 positive nodes without an associated decrease in receipt of reconstruction. This may represent increasing clinician comfort with irradiating a new breast reconstruction and may have cosmetic and quality-of-life implications for patients.