AbstractA case control study of 2,908 breast cancer cases and 3,180 controls, derived from a nationwide screening program, enabled evaluation of the relationship of breast cancer risk to a variety of menstrual factors. Risk was significantly inversely related to age at menarche, with women who first menstruated at age 15 or later having a 23% lower risk than those with menarche prior to the age of 12. There was a higher relative risk (1.3) for premenopausal than menopausal women. In contrast to previous studies, there was only a slight increase in risk associated with a late age at natural menopause, possibly owing to errors in recall. Bilateral oophorectomy at an early age exerted a protective influence on breast cancer risk, with effects manifested approximately 10 to 15 years after oophorectomy. Women who had both ovaries removed prior to the age of 40 had a 45% reduced risk compared to women with a natural menopause at ages 50 to 54. In addition, bilateral oophorectomy at an early age was associated with a lowered risk relative to natural menopause at a comparable age, which may reflect the more pronounced and sudden decline in endogenous hormones associated with the surgery. Although these results were based on patient reports regarding the types of surgical menopause experienced, validation against medical records showed close correspondence regarding the number of ovaries removed.