During a recent international workshop on carcinoma in situ and cancer of the testis held in November 1992, there was consensus about the management of the contralateral testis in patients with unilateral testicular cancer [20]. Such men should be offered a biopsy of the contralateral testis. This consensus was based on (1) the high prevalence of carcinoma in situ (CIS) in the contralateral testis found not only in Danish men but also in studies from other countries and (2) the possibility of eradicating the CIS and thereby preventing the development of invasive cancer by means of localized irradiation with preservation of testosterone production in the testis. The aim of this paper is to outline the present status of the radiation treatment of CIS of the testis.