CIS is found in the contralateral testis in 5% of the patients with testicular germ cell cancer. The management of CIS in the contralateral testis is important because the majority - if not all - cases of CIS will progress to invasive disease without treatment. It is well documented that testicular irradiation with a total dose of 14-20 Gy (2 Gy x 7-10) is an effective and safe treatment for CIS in the contralateral testis in patients with unilateral testicular germ cell cancer. However few relapses of testicular cancer have been observed in testis treated with these regimens and the data on 14 Gy are sparse. One study has indicated that more radiotherapy with lower doses per fraction could be useful, but more data are needed to confirm this. Endocrine testicular function has been shown to be impaired already before treatment in patients with CIS and is further impaired after testicular irradiation with 14-20 Gy (2 Gy x 7-10) and only minor dose dependency is seen in the impairment of Leydig cell function. The optimal treatment of CIS in the contralateral testicle in patients orchidectomised for testicular cancer seems to be local radiotherapy of the testis with CIS in order to preserve at least a part of the Leydig cell function. However, the optimal dose level has to be defined.