Radiotherapy can cause infertility in both men and women. However, few data are available concerning the effects of radioiodine therapy for thyroid carcinoma on testicular function. We investigated 25 men (age 23-73 years) with differentiated thyroid carcinoma in a longitudinal prospective trial. Follicle-stimulating hormone (FSH), inhibin B, luteinising hormone (LH) and testosterone were measured before (n = 25) and 3 months (n = 11), 6 months (n = 18), 12 months (n = 22), and 18 months (n = 18) after radioiodine therapy [radioiodine dose (mean +/- SEM): 9.8+/-0.89 GBq]. Before therapy, FSH was 5.4+/-0.77 IU/l; it increased significantly (P<0.001) to 21.3+/-2.4 IU/l after 6 months and fell to 7.4+/-1.3 IU/l after 18 months (normal range: 1.8-9.2 IU/l). Inhibin B was significantly decreased (P<0.001) from 178+/-25.3 pg/ml before therapy to 22.2+/-5.5 pg/ml after 3 and 29.4+/-5.7 pg/ml after 6 months and rose to 154+/-23.3 pg/ml after 18 months (normal range 75-350 pg/ml). LH and testosterone were within the normal range during the whole study (1.6-9.2 IU/l and 10.4-34.7 nmol/l, respectively). LH was significantly increased (P<0.001) from 2.8+/-0.33 IU/l before therapy to 5.9+/-0.69 IU/l 6 months after therapy and then fell slowly to 4.0+/-0.45 IU/l after 18 months. Total testosterone was significantly increased (P<0.01) from 12.8+/-0.99 nmol/l at baseline to 19.8+/-1.7 nmol/l after 12 months and 19.6+/-1.7 nmol/l after 18 months. The testosterone/LH ratio (normal range: 3.3-17.9 nmol/IU) fell from 5.8+/-0.66 nmol/IU to 3.0+/-0.36 nmol/IU after 3 months (P<0.01); it remained close to the latter value after 6 months (3.4+/-0.49 nmol/IU) and then rose to 5.5+/-0.6 nmol/IU after 18 months. In conclusion, 3 and 6 months after radioiodine therapy all patients showed elevated FSH and decreased inhibin B levels, reflecting severely impaired spermatogenesis. At the same time a compensated insufficiency of the Leydig cell function was observed. Eighteen months after the last radioiodine therapy, mean values of gonadal function had completely recovered.