BACKGROUND: Iodine-125 low-dose-rate brachytherapy (LDR-BT) is a treatment modality utilized in both localized and advanced prostate cancer (PCa). We aimed to evaluate the long-term oncological outcomes in patients with PCa who underwent LDR-BT, at a single institution in Japan. METHODS: We retrospectively reviewed the clinical records of 340 consecutive patients with localized PCa who underwent LDR-BT between August 2004 and December 2014 at our institution. Patients with low-risk PCa who had a pretreatment prostate volume >50 mL received neoadjuvant androgen deprivation therapy (ADT) for at least 3 months before LDR-BT. Patients with intermediate-risk PCa were treated with a combination of LDR-BT and/or external beam radiation therapy (EBRT) and/or ADT for 9 months. Patients with high-risk PCa underwent LDR-BT, EBRT, and ADT for 24 months. The endpoints of this study were biochemical recurrence-free survival (BRFS) and overall survival (OS). Additionally, the association between biochemical recurrence (BCR) and clinical/pathological covariates was analyzed. RESULTS: At the end of the follow-up period, nine patients (2.6%) showed BCR, and six patients (1.8%) developed secondary cancers after LDR-BT. The 5-year and 10-year BRFS rates were 99.4% and 95.3%, respectively. Factoring in the patients' ages, the 5-year and 10-year BRFS rates were 99.1% and 99.1%, respectively, in patients aged >63 years. The rates were 100% and 89.4% in those aged ≤63 years, respectively. In the multivariate analysis, age ≤63 years was identified as a significant independent predictor of BCR after LDR-BT. CONCLUSION: Age ≤63 years was a significant predictor of BCR following LDR-BT. Although the risk of secondary malignant neoplasms should be considered when opting for LDR-BT in younger patients with PCa, the prevalence of them in these patients is relatively low. Therefore, clinicians should weigh the risks and benefits of definitive therapy in PCa, particularly in younger patients.