The aim of this study was to evaluate the effect of treatment with radioactive iodine ((131)I) on gonadal function in males and females with follicular or papillary thyroid carcinoma. Consenting patients at reproductive age were grouped according to the cumulative dose of (131)I received and followed-up for at least 12 months. Overall, 246 patients (159 females, 87 males) were studied. In all males, serum levels of follicle-stimulating hormone (FSH), luteinising hormone (LH), and testosterone were measured before radioiodine treatment and 2, 6 and 12 months afterwards; fifty-three of all patients underwent semen analysis also. On females, tests for serum levels of LH, FSH, estrogen, and progesterone were performed. In 87.4% of males, there was an increase in serum FSH level after (131)I treatment and in 20.7% of them this level remained high during the follow-up period. The average serum level of FSH 2-6 months after each course of treatment was significantly higher than before treatment (p<0.01), and there was a significant correlation with the cumulative dose of (131)I received (p<0.001). Reduced sperm count was found in 35.8% of the male patients, among whom 73.7% also showed reduced motility. In 36.8% of the patients with reduced sperm count (13.2% of the total), this finding was persistent during the follow-up period. Increased level of FSH was correlated with reduced sperm count in all doses (p<0.005). There was no significant correlation between serum levels of LH and testosterone with (131)I treatment in males. In females, no significant correlation between gonadal-hypophyseal hormones and treatment with (131)I was found, and there were no signs and symptoms of sexual dysfunction. Infertility was not noticed in any patient neither was there any case of abortion. Although female gonads are resistant to radioiodine treatment for thyroid cancer, in males this treatment may result in impairment of the gonadal function, which is transient most of the times. Spermatogenesis is especially sensitive to the radiation effect of (131)I treatment, and this effect is related to the cumulative radioiodine dose. To reduce gonadal complications, especially in males, treatment with (131)I should use the lowest possible doses. Also all necessary measures should be taken to reduce radiation dose to gonads.