Abstract:  More women die of cardiovascular disease than the next seven causes of death combined. While the rate of cardiovascular disease has been steadily declining in men, it has remained relatively constant in women. However, most drugs and devices used to treat heart disease have been studied in men and then applied to women, assuming that doses and devices should be scaled down, purely by size. Medical treatment should take biologic sex/gender into consideration. For example, women having a heart attack may have different symptoms than men.Those symptoms in women might include fatigue, body aches and jaw pain. In support of these sex differences, low doses of aspirin are beneficial for both sexes but in women, low-dose aspirin helps prevent stroke but not heart attacks. In men, low-dose aspirin helps prevent heart attacks but not stroke. These facts emphasize the inadequacy of studying drugs and devices only in men. Researchers are now beginning to conduct studies that include female animals and women.This is a welcome change given that sex differences extend to many arenas, including responses to exercise and to the effect of cancer on the heart. For instance, male mice voluntarily run 25 to 31 miles per week. But female mice run twice the distance of males each week, and they have stronger cardiac responses to a given amount of exercise than males. Similarly, hearts of male animals response much more pathologically than hearts of females to cancer. These differences have been studied in mice and observed in humans. Males with cancer lose more body weight and cardiac mass than do females; and both of these are detrimental. This difference is due to higher levels of estrogen in females. When estrogen receptors are blocked in female mice, they experience weight loss and heart-mass loss similar to males. At the same time, if male mice are given estrogen, they get worse, indicating that estrogen might be protective in females, but harmful in males. This could be relevant to humans who ingest large quantities of soy (Americans spent $8 billion on soy products in 2008), which contain large quantities of plant estrogens. Normal laboratory mouse food is soy-based, and male mice fed this diet have much worse heart disease than do female mice. Taken together these facts suggest that soy consumption in laboratory animals may influence functional outcomes and that sex-based medicine should be an important consideration.