Carcinoma in situ in the contralateral testis was diagnosed in 27 of 500 patients (5.4%) with unilateral testicular germ cell cancer. Eight of the 27 patients received intensive chemotherapy for spread of their initial testicular cancer. Follow up biopsy studies did not detect changes of carcinoma in situ in any of these patients, and none developed a contralateral testicular tumour (observation time 12-88 months). Of the remaining 19 patients with carcinoma in situ, seven developed contralateral testicular cancer. The estimated risk of developing invasive growth was 40% within three years and 50% within five years. None of the 473 patients without carcinoma in situ detected by screening biopsy developed contralateral testicular cancer (observation time 12-96 months). No serious complications arose from the biopsy procedures. All patients with unilateral testicular germ cell cancer should be offered biopsy of the contralateral testis.