A 40-year-old man with Down's syndrome was admitted to our hospital for complaint of painless, swollen right scrotum. Computed tomography revealed a right testicular tumor and mass of iliac fossa. Right high orchiectomy and extirpation of mass were performed. Histological diagnosis of right testicular tumor was seminoma and the mass was an external iliac lymph node metastasis of seminoma. After operation, radiation therapy was performed.