Purpose: To evaluate the oncological outcomes and genitourinary and gastrointestinal adverse events in acute and late-phases of iodine-125 low-dose-rate brachytherapy for localized prostate cancer. Material and methods: We retrospectively evaluated 334 patients treated for localized prostate cancer with low-dose-rate brachytherapy. Bio-chemical relapse-free survival, cause-specific survival, and overall survival were evaluated using Kaplan-Meier method and log-rank test. Incidence of adverse events was calculated using National Cancer Institute common terminology criteria for adverse events, version 5. Logistic regression was used to identify independent predictors of acute and late-phase genitourinary and gastrointestinal adverse events. Results: National Comprehensive Cancer Network's low-, intermediate-, and high-risk groups included 133 (39.8%), 163 (48.8%), and 38 (11.3%) patients, respectively. The 5-year cause-specific survival rate was 100%. The 5-year bio-chemical relapse-free survival rates for the low-, intermediate-, and high-risk groups were 98.3%, 95.8%, and 100%, respectively. One patient had a ≥ grade 3 acute adverse event. The 5-year cumulative ≥ grade 1, ≥ grade 2, and ≥ grade 3 genitourinary adverse event rates were 27.9%, 14.4%, and 0.5%, respectively. The 5-year cumulative ≥ grade 1, ≥ grade 2, and ≥ grade 3 gastrointestinal adverse event rates were 3.1%, 1.5%, and 0.5%, respectively. A high pre-treatment international prostate symptom score and non-use of α1-blockers were associated with an increased risk of acute genitourinary adverse events. Conclusions: Low-dose-rate brachytherapy had good oncological outcomes, with acceptable adverse event rates. Pre-treatment urinary function and use of α1-blockers may be useful in predicting and preventing acute genitourinary adverse events.