© 2019 The Author(s).Background: Aging is significantly related to multiple comorbidities. Even with a good performance score, some elderly patients may have poor survival outcomes. We aimed to evaluate the prognostic value of the Charlson comorbidity index (CCI) for mortality and toxicity in elderly patients with locally advanced rectal cancer (LARC). Methods: Seventy-two elderly patients with LARC who were treated with neoadjuvant chemoradiotherapy (CRT) were included. Based on their CCI score, severity of the comorbidity was categorized into 2 groups: CCI<7 and CCI≥7. Results: The overall survival (OS) at 5 years was 54.4 percent in patients treated with neoadjuvant CRT. Median OS was not reached for all patients as well as patients with CCI score <7, but median OS was 25 (95% CI 1.0–62.1) months in patients with CCI≥7 (P = 0.002). The OS at 2 years was 79.1 percent in the patients with CCI <7 and 50.0 percent in the patients with CCI score ≥7 (P = 0.002). Moreover, there was a trend toward, patients with higher CCI score who had more treatment related to grade 3 or 4 toxicity compared to those with CCI score <7 (33.3% vs 13.3%, respectively, P = 0.09). Multivariable analysis indicated that the CCI score ≥7, presence of down-staging after therapy and clinical stage (III) independently predict mortality (HR 6.14, 95%CI 2.45–15.35, P < 0.001) in patients with LARC. Conclusion: Although CCI score was not significantly associated with both toxicity and disease-free survival (DFS), we suggest that baseline CCI score might be a valuable prognostic indicator for physicians to evaluate elderly patiens with LARC for optimal treatment.