Four patients with unilateral testicular germ-cell tumor and biopsy-proven contralateral testicular intraepithelial neoplasia (TIN; so-called carcinoma in situ) received localized low-dose radiation therapy (18-20 Gy) of the testis with TIN. Repeated biopsies disclosed the disappearance of TIN and germ cells. No recurrence of TIN or germ cells was observed after a follow-up of 18-42 months. All patients reported a normal sex life without need of androgen supplementation. Serum follicle-stimulating hormone increased significantly immediately after radiation treatment and started to decline after 24 months. Serum luteinizing hormone increased slightly but not significantly. A decline after 24 months was observed in only one of three patients. Serum testosterone decreased significantly in all patients after 1 year but never became subnormal. Low-dose radiation treatment is efficacious in eradicating testicular intraepithelial neoplasia without destroying Leydig cells or stromal cells of the testis. Thus, a patient otherwise destined to develop a second testicular tumor can be spared orchiectomy and life-long hormonal replacement.