PURPOSE: To determine the effect of radiotherapy in doses 14 to 20 Gy on eradication of carcinoma-in-situ (CIS) testis and on the Leydig cell function. PATIENTS AND METHODS: Forty-eight patients presented with unilateral testicular germ cell cancer and CIS of the contralateral testis. The CIS-bearing testis was treated with daily irradiation doses of 2 Gy, 5 days a week, to a cumulative dose of 20 Gy (21 patients), 18 Gy (three patients), 16 Gy (10 patients), and 14 Gy (14 patients). RESULTS: All patients treated at dose levels 20 Gy to 16 Gy achieved histologically verified complete remission without signs of recurrence of CIS after an observation period of more than 5 years. One of 14 patients treated at dose level 14 Gy had a relapse of CIS 20 months after irradiation. Leydig cell function was examined before and regularly after radiotherapy in 44 of 48 patients. The levels of testosterone were lower after radiotherapy than before. Testosterone showed a stable decrease for more than 5 years after treatment (3.6% per year) without dose dependency. The levels of luteinizing hormone and follicle-stimulating hormone were increased after radiotherapy. The need of androgen substitution therapy was similar at all dose levels. CONCLUSION: Testicular irradiation is a safe treatment at dose level 20 Gy (10 x 2 Gy). Decrease of dose to 14 Gy (7 x 2 Gy) might lead to risk of relapse of CIS. Impairment of hormone production without clinically significant dose dependency is seen in the dose range 14 to 20 Gy.