Five cases of cancer of the testicle in patients with cryptorchidism are presented. In one case the tumour occurred 16 years after cure for cryptorchidism at 15 years of age. In the other cases, the tumour developed on a testicle in an inguinal or intra-abdominal position. In two cases, it was impossible to remove the tumour which had developed on an intra-abdominal position. These patients died. These cases emphasize the importance of surgical cure in all cases of cryptorchidism, preferable before the age of 2 years, in order to improve functional prognosis and reduce the risk of cancerization. The gravity of cancer of the testicle in cases of cryptorchidism is often discovered too late.