Background: Invasive procedures including biopsies in freshly irradiated tissue is a known risk factor for high-grade toxicity. Screening colonoscopies (CS) are often performed before radiotherapy given the need to rule out comorbid gastrointestinal conditions and avoid untoward post radiotherapy instrumentation of the pelvis. We investigate the association of pre-treatment CS and the colonic finding therein with subsequent GI toxicity following SBRT for prostate cancer. Methods: An institutional registry of patients undergoing five-fraction prostate SBRT was interrogated to identify those who underwent screening CS prior to radiotherapy from Feb. 2021 – May 2023. Patients were categorized into those who did and did not undergo CS within 6 months of SBRT. A detailed analysis of CS findings including polyp resection as well as presence of diverticulosis and hemorrhoids was performed. Post-SBRT toxicity was evaluated using CTCAE v 5.0. Groups were compared using the chi-square or Fisher’s exact test for categorical variables. The Mann-Whitney test was used to compare groups for time from CS to SBRT and presented as median (25^th, 75^th percentiles). A result was considered significant at p < 0.05. Results: In this cohort, a total of 156 patients underwent prostate SBRT with the distribution of risk grouping was as follows: low 9% (n = 14), intermediate 67% (n = 104), and high 24% (n = 38). Of the entire group, a total of 138 patients underwent pre-treatment CS with a median time from CS to SBRT of 4 months. There was no difference in grade 1+ GI toxicity (67% vs. 61%, p = 0.60) in patients who did and did not undergo pretreatment CS. However, there was a significantly higher grade 2+ GI toxicity in patients who did not undergo pretreatment CS (45% vs. 15%, p = 0.03). Of the 138 subjects who underwent CS, time from CS to SBRT was not different between those who did and did not have any GI toxicity (4.6 months versus 3.3 months, p = 0.08) or between those who had grade 1 vs. grade 2+ toxicity (4.6 versus 4.7 months, p = 0.65). Notably, there was not an increased rate of GI toxicity in patients found to have diverticulosis (61.1% vs. 74.2%, p = 0.11), hemorrhoids (62.3% vs. 70.6%, p = 0.31), or resected polyps (70% vs. 64.7%, p = 0.51) on pre-SBRT CS. Finally, patients who had polyps resected in the rectosigmoid did not have a higher rate of GI toxicity versus those who under polyp resection elsewhere in the colon (57.6% vs. 71.4% respectively, p = 0.23). Conclusions: Pretreatment CS was not associated with an increased risk of GI toxicity following prostate SBRT, and in fact was associated with a lower rate of grade 2+ GI toxicity. Moreover, identification of polyps requiring resection (in the rectosigmoid or elsewhere), diverticulosis, and hemorrhoids on pretreatment CS did not result in excess GI toxicity following SBRT.