The incidence of a second primary testicular germ cell cancer among 2850 (96.6% of eligible) men with a histologically verified first primary germ cell cancer diagnosed in the period 1960-1979 in Denmark was established. Of these 2850 men, 73 (2.6%) developed a contralateral testicular cancer. In five of these patients (0.18%), the tumors were synchronous. The cumulative risk of developing a contralateral cancer 25 years after diagnosis of the first testicular germ cell cancer was 5.2% according to a Kaplan-Meier estimate. It was higher among men with a nonseminoma as the first tumor (8.4%) than among men with a seminoma as the first tumor (3.6%). Of the second tumors, 12% were stage II and 17% were stage III at the time of diagnosis. Based on 24,588 person-years at risk and 68 nonsimultaneously occurring bilateral testicular germ cell cancers, the overall relative risk (RR) of developing a second primary cancer in the contralateral testicle following a first germ cell cancer was found to be 24.8 (95% confidence interval = 19-38). Among men with a nonseminoma, the risk was higher (RR = 27.1) than among men with a seminoma (RR = 22.5). The excess risk was not affected by age at diagnosis, calendar period, or time since diagnosis. Close surveillance by screening for and treatment of carcinoma in situ of the remaining testicle in testicular cancer patients are advised.