The role of iodine-131 therapy in the management of differentiated thyroid carcinoma is well established. Because the spermatogonia are very sensitive to radiation, there is concern that large doses of the latter could result in azoospermia and permanent infertility. For patients treated with a single ablation dose, testicular function recovers within months and the risk of infertility is diminished. Gonadal damage may be cumulative in those requiring multiple administrations. In all young male patients, but especially in those with metastatic or pelvic disease or both, the long-term storage of semen should be addressed prior to therapy. Sperm banking should be considered in patients likely to be given cumulative doses greater than 14 GBq of 131I. However, the patient's threshold for sperm banking might be even lower than that.