This article attempts to bridge the disciplinary gap between evolutionary biology and clinical studies of women's health. The resulting dialogue is predicted to have useful implications for research aimed at the prevention of women's reproductive cancers. The specific focus is on the relationship between breast cancer and exposure to ovarian hormones during normal menstrual cycling. The clinician's view of normal cycling is radically different from that uncovered by evolutionary studies of noncontracepting populations. This point is illustrated by data on the Dogon of Mali, a traditional West African population with a mean of 8.6 +/- 0.3 live births per woman. The Dogon data include hormonal profiles (urinary estrone-3-glucuronide and pregnanediol-3-glucuronide) of 93 women (sampled twice weekly for 10 weeks) and a census of women's visits to menstrual huts (n = 736 days). Dogon women menstruated regularly only if they were sterile. Otherwise, women aged 20-34 years had a median of only two menses each over the 2-year study period. The median number of menses per lifetime was approximately 100, about a third as many as experienced by an American woman who had three live births. These results contribute to a growing body of evidence that women's bodies were designed by natural selection to spend most of the time in lactational amenorrhea and add support to the view that contraceptives can be made safer if they forego the hormonal swings associated with menstruation. This conclusion is further reinforced by evidence that menstrual bleeding serves no adaptive purpose.