As part of the tumor aftercare service afforded by our policlinic, 91 patients were examined one to six times in 18 months following radical orchiectomy for testicular germ cell tumors. The period of observation averaged 35 months (3-147) after the contralateral removal of the testicle. In addition to the usual routine checks, sonography of the residual testicle also formed part of the established followup protocol. Among these patients sonography identified in residual testicles 3 testicular tumors (3.3%) which had escaped detection on palpation. The existence of these second tumors was demonstrated 3, 25 and 38 months following contralateral orchiectomy. In all instances the second lesion differed from the first tumor in histology. In another patient a plum-sized malignancy was revealed within a large concurrent hydrocele by scrotal sonography carried out 18 months following preceding contralateral orchiectomy. Thus, our group of patients gave an incidence of 4.4% for second primary testicular tumors, a poor known incidence rate. Such malignancies can be spotted early by ultrasound, they are genetically obscure just as the increasing morbidity of the unilateral disease. These results urgently suggest that after semicastration for testicular germ cell tumor the residual testicles should be regularly scanned by ultrasonography at four-month intervals. Retrograde sonography has been shown to detect even those small occult testicular tumors which pass unnoticed until metastasizing.