Many patients with Hodgkin's disease, acute leukemia, non-Hodgkin's lymphoma, testicular cancer, and other tumors now regularly achieve sustained clinical remissions and cures. Drugs used in the treatment of cancer have profound and often lasting effects on the testis and ovary. Germ cell production and endocrine function may both be altered with the magnitude of the effect related to the age, pubertal status, and menstrual status of the patient as well as to the particular drug, dosage, or combination administered. The primary testicular lesion caused by all antitumor agents studied thus far is depletion of the germinal epithelium lining the seminiferous tubules. Combination chemotherapy regimens that include alkylating agents produce germinal aplasia and permanent infertility in the majority of patients. The risk of ovarian injury following combination chemotherapy is clearly related to the age of the patient at the time of treatment. Overall, 40 to 50% of women treated with combination chemotherapy become amenorrheic, although the frequency of amenorrhea in women older than 35 years may be as high as 90%. Interventions to protect the gonads from the effects of chemotherapy have not yet been developed; thus, male patients should be offered an opportunity to store semen prior to treatment and all patients should be counseled concerning the potential gonadal toxicity of cancer chemotherapy.