Background: Acute toxicity may be a factor interfering with receipt of chemoradiation (CRT) therapy for rectal cancer. The purpose of this study was to identify clinical and treatment factors associated with increased acute toxicity in patients receiving CRT therapy for rectal cancer. Methods: We identified patients with rectal adenocarcinoma treated with CRT between 2006-2014 at an NCI-designated cancer center. Patients with metastatic disease or missing treatment information were excluded. Acute toxicity information including weight loss, pain, fatigue, constipation, diarrhea, anorexia, and performance status was extracted from weekly on treatment visit notes. Multivariable logistic regression was used to assess predictors of grade 3+ toxicity using covariates significant on univariable analysis. Results: A total of 148 patients were included with a median age of 59 (range 29-99). The majority of patients were male (55%) and received 5-FU based chemotherapy (82%). During CRT, 35 (24%) patients experienced at least one grade 3+ toxicity: 13 (9%) patients experienced grade 3+ fatigue, 1 (1%) experienced grade 3+ constipation, 11 (7%) experienced grade 3+ diarrhea, 14 (10%) experienced grade 3+ pain, and 11 (7%) experienced grade 3+ anorexia. Eight (5.4%) patients had an ECOG performance status > 3 and 28 (19%) patients had weight loss > 5 lbs during CRT. On multivariable analysis, increased distance from the anal verge (OR 0.78 95% CI 0.636-0.998) was associated with a decreased risk for grade 3+ pain and age > 75 was associated with an increased risk of grade 3+ anorexia (OR 6.07 95% CI 1.067-34.56). Clinical T4 disease was associated with an increased risk of weight loss > 5 lbs (OR 0.17 95% CI 0.100-0.446). On multivariable analysis, there were no factors associated with grade 3+ fatigue, diarrhea, or constipation. There were no factors associated with a decline in performance status to > 3 while on treatment. Conclusions: Our results suggest that rectal cancer patients who are older, have more advanced disease, or with low lying tumors may be at an increased risk for treatment-related toxicity. Identifying predictors of toxicity may allow for tailored interventions to minimize toxicity for these patients.