Epidemiological studies have consistently found that bilateral oophorectomy at a young age substantially reduces breast cancer risk. Such surgical menopause around age 35 has been found to reduce risk by 60 to 75%. A reversible medical oophorectomy using an agent such as a gonadotropin-releasing hormone agonist (GnRHA) should achieve a similar reduction in risk. Although the use of GnRHA alone is unacceptable because of the associated hypoestrogenic side-effects, these can be satisfactorily prevented by add-back low-dose estrogen treatment with intermittent progestin to protect the endometrium. It is estimated that a regimen of GnRHA plus add-back ultra low-dose estrogen and progestin would prevent some two-thirds of current breast cancer if used from age 30. If used from age 20 almost nine out of ten current breast cancer cases would be avoided. If, as is likely, these estimates also apply to women at high genetic risk of breast cancer because of possession of a BRCA1 or BRCA2 gene, their breast cancer risk would be reduced to below that of 'normal' women. The protective effects on ovarian cancer are calculated to be greater than the protective effects on breast cancer. Practical chemoprevention of breast and ovarian cancer using this approach should be possible within 5 years.