One year has passed since we started brachytherapy with radioactive iodine-125 seeds for carcinoma of the prostate. During the follow-up of patients, we have relatively frequently found migrated seeds in the lungs. Migrated seeds are reported to reach mainly the pulmonary artery and cause embolization without clinical symptoms. We counted the embolized seeds and determined the proportion of migrating seeds on chest X-ray exam. We found 47 cases of pulmonary embolization in our initial 100 cases. Less than half of the embolization were found in the chest X-ray exam performed on the next day after the implantation. We found more migrated seeds in the lower lung fields than in the upper and middle lung fields. Pulmonary embolization of implanted iodine-125 seeds is not unusual, and cases of prostate brachytherapy are likely to increase in Japan. We will have increased opportunities to observe chest X-ray films with migrated seeds in the future.