The purpose of this study was to determine the risk factors for rectal bleeding after prostate brachytherapy. Between April 2005 and September 2009, 89 patients with T1c-2cN0M0 prostate cancer were treated with permanent I-125 seed implantation alone. The prostate prescription dose was 145 Gy, and the grade of rectal bleeding was scored according to the Common Terminology Criteria for Adverse Events version 4.0. Post-treatment planning was performed with fusion images of computerized tomography and magnetic resonance imaging 4-5 weeks after brachytherapy. Patient characteristics and dosimetric parameters were evaluated to determine risk factors for bleeding. The calculated parameters included the rectal volume in cubic centimeters that received >50-200% of the prescribed dose (RV50-200) and the minimal doses received by 1-30% of the rectal volume (RD1-30). The median follow-up time was 42 months (ranging 18-73 months). Grade 1 rectal bleeding occurred in 24 (27.0%) patients, but no Grade 2 or severe bleeding was observed. Usage of anticoagulants had a significant correlation with the occurrence of bleeding (P = 0.007). The RV100-150 and RD1-10 were significantly higher in patients with rectal bleeding than in those without bleeding. The RV100 was identified as a possible threshold value; the 3-year rectal bleeding rate in patients with an RV100 > 1.0 cm(3) was 36%, whereas that with an RV100 ≤ 1.0 cm(3) was 14% (P < 0.05). Although no Grade 2 morbidity developed in this study, the RV100 should be kept below 1.0 cm(3), especially in additional dose-escalated brachytherapy.