OBJECTIVE We report two cases of seed migration to the vertebral venous plexus after iodine-125 (I-125) transperineal interstitial permanent prostate brachytherapy. METHODS Case 1: A 67-year-old Japanese man underwent percutaneous transperineal interstitial permanent prostate brachytherapy at our institution. Three months after brachytherapy, routine followup kidney-urinary bladder (KUB) radiography showed two seeds that had migrated to the pelvic area and were overlapped by sacral bone. It was very difficult to detect the seeds by visceral CT, because seeds were in contact with to vertebral bone, and seeds and bone were of the same CT value in visceral CT. But bone CT could distinguish seeds and bone, and it showed seed migration to the vertebral venous plexus in the sacral vertebral canal. Case 2: A 75-year-old Japanese man underwent percutaneous transperineal interstitial permanent prostate brachytherapy at our institution. The day after seed implantation, routine followup KUB radiography showed that a seed had migrated to the pelvic area and was overlapped by sacral bone. Bone CT clearly showed seed migration to the vertebral venous plexus in the vertebral canal in comparison with visceral CT. RESULTS Seeds that have migrated to the vertebral venous plexus are difficult to be detected by visceral CT or KUB radiography. In visceral CT, it is difficult to distinguish seed and bone, especially when they are touching each other because they have the same CT value in visceral CT. It is therefore necessary to perform bone CT to detect such migrating seeds. CONCLUSIONS To our knowledge, this is the first report of seed migration to the vertebral venous plexus after prostate brachytherapy. We thought that seeds migrate to the vertebral plexus via the pelvic venous pathway. If seed migration to the pelvic area and the overlapped sacral bone area is found after brachytherapy, bone CT should be performed, especially when it is difficult to detect the seed in visceral CT.