Although tumors of the testis may originate from several types of cells in the seminiferous tubules or the interstitial tissue, most testicular tumors are germ-cell neoplasms.1 Approximately 25% to 30% of all germ-cell tumors produce human chorionic gonadotropin (hCG), α-fetoprotein (AFP), or both. Tumors originating from Leydig cells or Sertoli cells often produce sex steroids (testosterone or estrogen). Therefore, germ-cell tumors as well as Leydig- and Sertoli-cell neoplasms may be associated with clinical signs of abnormal hormone production (e.g., gynecomastia; Fig. 122-1). Moreover, testicular cancer is associated with an increased risk of infertility. Hence, these lesions are of considerable importance to the clinical endocrinologist.