As survival rates for childhood cancer have improved, the importance of assessing gonadal dysfunction caused by alkylating agents and radiotherapy in children treated for cancer has increased. Infertility is the major long-term side effect of chemotherapy (CT) in males, whereas Leydig cell function is less affected. Our studies confirm that prepuberty does not protect the male gonad from the late effects of CT and that protocols less gonadal-lesive (such as ABVD regimens) should be preferred. Ovaries are less affected, but early depletion of follicles and premature menopause may occur. High-dose busulfan conditioning regimens cause ovarian failure in young females. The role of gonadal irradiation is discussed: high dosages (>2000 cGy) provoke sterility, impaired testosterone secretion in males and estradiol release in females. High dosage hypothalamic-pituitary irradiation causes delayed puberty and hypogonadism in males and females, whereas lower dosages may be associated with early puberty, particularly in females.