The incidence of a second primary testicular germ cell cancer in the contralateral testicle among 2338 men with a first primary testicular germ cell cancer diagnosed in the years 1960-79 in Denmark was established in this preliminary report. The material represents 83% of the total cohort followed until 31 December 1984. The relative risk for a patient with testicular cancer to get yet another testicular cancer was studied, taking into account the histology of the first primary testicular germ cell cancer. Based on fifty-eight nonsimultaneous contralateral testicular cancer cases and 19,995 'person-years at risk', the overall relative risk of invasive germ cell cancer in the contralateral testicle following a first germ cell testicular cancer was found to be 23.3 (95% confidence interval: 18-30). Among men with nonseminoma the risk was higher (relative risk = 27.5) than among men with seminomas (relative risk = 20.1). Overall, sixty-two (2.7%) patients developed a second cancer. In four of these patients bilateral tumours occurred simultaneously.