The hypothalamic-pituitary-testicular axis finely regulates levels of circulating sex steroids--especially testosterone and estradiol--and spermatogenesis. Testosterone, directly as an androgen and as a prehormone for estradiol, regulates LH secretion at both hypothalamic and pituitary levels. Leydig cells, principally under the control of LH, produce testosterone. Sertoli cells, under the control of FSH, and sensitive to intratesticular levels of testosterone, produce estradiol. This locally produced estrogen seems to be necessary for maturation of the germ cells. An abnormality in this sensitive control system, leading to elevations in gonadotrophins or steroid levels, may be etiologically important in both germ cell and nongerm cell neoplasia. Testicular cancers are associated frequently with endocrinologic manifestations, which may be more disabling to the patient than the malignant potential of the tumor, especially with childhood Leydig cell tumors. Estrogen dominance with an elevated estrogen/testosterone ratio can be seen in any testicular neoplasm and may result in gynecomastia. It may be due to a decrease in circulating testosterone or to an increase in estrogens. Virilization is seen frequently in Leydig cell tumors of adolescents. Further elucidation of hormonal interrelationships should lead to better understanding of the genesis of testicular neoplasia and to more effective therapy.