PURPOSE: Historically, toxicity concerns have existed in patients with large prostate glands treated with radiation therapy, particularly brachytherapy. There are questions whether this risk extends to stereotactic body radiation therapy (SBRT). In this retrospective review, we examine clinical outcomes of patients with prostate glands ≥100 cc treated curatively with SBRT. METHODS AND MATERIALS: We retrospectively analyzed a large institutional database to identify patients with histologically confirmed localized prostate cancer in glands ≥100 cc, who were treated with definitive-robotic SBRT. Prostate volume (PV) was determined by treatment planning magnetic resonance imaging. Toxicity was measured using Common Terminology Criteria for Adverse Events, version 5.0. Many patients received the Expanded Prostate Cancer Index Composite Quality of Life questionnaires. Minimum follow-up (FU) was 2 years. RESULTS: Seventy-one patients were identified with PV ≥100 cc. Most had grade group (GG) 1 or 2 (41% and 37%, respectively) disease. All patients received a total dose of 3500 to 3625 cGy in 5 fractions. A minority (27%) received androgen deprivation therapy (ADT), which was used for gland size downsizing in only 10% of cases. Nearly half (45%) were taking GU medications for urinary dysfunction before RT. Median toxicity FU was 4.0 years. Two-year rates of grade 1+ genitourinary (GU), grade 1+ gastrointestinal (GI), and grade 2+ GU toxicity were 43.5%, 15.9%, and 30.4%, respectively. Total grade 3 GU toxicities were very limited (2.8%). There were no grade 3 GI toxicities. On logistic regression analysis, pretreatment use of GU medications was significantly associated with increased rate of grade 2+ GU toxicity (odds ratio, 3.19; P = .024). Furthermore, PV (analyzed as a continuous variable) did not have an effect on toxicity, quality of life, or oncologic outcomes. CONCLUSIONS: With early FU, ultra large prostate glands do not portend increased risk of high-grade toxicity after SBRT but likely carry an elevated risk of low-grade GU toxicity.