Acute lymphoblastic leukemia is by far the most frequent malignant disease in children. In all, 5% of the boys affected will develop testicular disease either at initial presentation or during the disease course or as the first site of relapse. Modern treatment regimens have reduced the occurrence of testicular relapses, which was more frequent in the 1970s. There is no place for preventive measures for early recognition of testicular leukemia; routine biopsies have been abandoned, and prophylactic irradiation is not justified. In gross overt disease, orchiectomy is justified (1) in cases of huge bulky testicular disease, (2) if unilateral disease is probable, and (3) if radiation of the testes is refused. In malignant non-Hodgkin's lymphoma, orchiectomy may eventually be the best mode of diagnosing the disease if a boy presents with testicular enlargement. Standard local treatment, however, is irradiation of both testes, if both are affected.