Background: Guidelines for stage II and III rectal cancer in the US recommend neoadjuvant chemoradiation therapy (CRT), curative resection, and adjuvant chemotherapy. This paradigm is based on early trials without neoadjuvant CRT and extrapolations from colon cancer. Recent trials have called into question whether adjuvant chemotherapy improves overall survival (OS) among patients treated with neoadjuvant CRT. We sought to examine whether chemotherapy improves OS in patients treated with neoadjuvant CRT or radiation therapy (RT) and surgery in real world settings. Methods: We identified a population-based cohort of 1,431 older (65+ years) non-metastatic rectal cancer patients diagnosed from 2004-2009 using the Surveillance, Epidemiology and End Results program (SEER)-Medicare data, who underwent neoadjuvant CRT or RT and curative resection. We described patterns of adjuvant chemotherapy using binomial regression models and evaluated the comparative effectiveness of: 1) any adjuvant chemotherapy vs. no adjuvant chemotherapy and 2) adjuvant oxaliplatin+5-fluorouracil (5-FU)/capecitabine vs. 5-FU/capecitabine on OS using Cox proportional hazards regression models and propensity score (PS) matching to adjust for measured covariates. Results: In total, 744 patients (52%) received adjuvant chemotherapy; most received oxaliplatin (53%). Older age, lower pathologic stage, and being widowed were associated with a lower likelihood of receiving adjuvant chemotherapy. Unadjusted and PS adjusted survival was superior in patients treated with chemotherapy (adjusted hazard ratio (aHR)=0.71, 95% confidence interval (CI): 0.57, 0.89). Among patients receiving adjuvant therapy, older age, earlier year of diagnosis, lower stage, and higher census tract poverty level were associated with a lower likelihood of receiving oxaliplatin. The addition of oxaliplatin to 5-FU/capecitabine did not improve OS after PS matching (aHR=1.15, 95% CI: 0.78, 1.70). Conclusions: Our results suggest that older non-metastatic rectal cancer patients benefit from adjuvant chemotherapy; however, the addition of oxaliplatin to 5-FU/capecitabine may not provide any incremental benefit.