A central epidemiological feature of cancers of the breast, endometrium and ovary is the sharp slowing down in their rate of increase with age around the time of menopause. The incidence of these tumors by the age of 70 years would be between fourfold and eightfold increased if the rapid increase with age seen in young women continued into old age. These phenomena can be explained by the different effects of ovarian hormones on cell division rates in the relevant tissues. Models of these effects provide a plausible explanation of most of the known epidemiology of each of the cancers, including the increase in breast cancer risk from menopausal estrogen-progestin therapy. Some recent epidemiological findings in endometrial and ovarian cancer suggest new avenues for possible chemoprevention of these cancers.