Before the introduction of effective chemotherapy for testicular germ cell tumours, the relative frequency of bilateral germ cell tumours was about 1.6%. Now, as more patients with unilateral tumours survive, an increase in this percentage is to be expected. We therefore compared a series of 397 patients with testicular germ cell tumours observed between 1950-1975 with a series of 333 patients with such tumours observed between 1976-1985, after the introduction of chemotherapy with cis-platinum. In both series 7 patients had bilateral germ cell tumours, i.e. 1.8% of cases in the first series and 2.1% of cases in the second series. Since the second series of cases with a much shorter follow-up has an unbalanced ratio of seminomatous to non-seminomatous tumours, an increase in bilaterality can be expected. The prophylactic and therapeutic implications of an increase in bilateral germ cell tumours of the testis are discussed.