The authors report a retrospective review of their experience with bilateral testicular cancers over two 10-year periods, one each from the prechemotherapy era (1935-1944) and the postchemotherapy era (1977-1986). Three of 295 patients (1.02%) evaluated at Mayo Clinic (Rochester, MN) for testicular germ cell malignancy between 1935 and 1944 had evidence of bilateral testicular malignancy. Two of these were synchronous and one metachronous occurring 3 years after the first diagnosis. In all three, the histology was pure seminoma. None of these three had a history of undescended testes. Both patients with synchronous tumours died within 2 years (6 months and 2 years, respectively) in spite of appropriate treatment at that time, and the one with metachronous tumor survived long-term (47 years). During the modern chemotherapy era, 16 of 500 (3.2%) patients evaluated at the Mayo Clinic Rochester between 1977 and 1986 for testicular germ cell malignancy had evidence of bilateral testicular cancers (four of these were synchronous and 12 metachronous [eight seminomas, four non-seminomas]) occurring between 1 to 15 years after the first diagnosis. Only two of 16 had a history of undescended testes surgically corrected while the patients were in their teens. All patients did well after appropriate treatment. This study reemphasizes the small but definite risk for development of a second testicular malignancy and suggests a recent increase in incidence of bilateral testicular tumors as possibly related to improved treatment modalities with a higher cure rate of the original tumor.