In the past, single-sex research on women was nearly always related to the reproductive system, while research performed on men encompassed ailments affecting both genders in the rest of the human body. That research, and trials including both sexes but not analyzed by gender, has been the basis for the care of both men and women. However, women differ from men not only in anatomy and reproductive physiology but also in general disease etiology, symptom profiles, and response to treatment—not to mention crucial psychosocial variables. A world view blind to those differences deprives scientists of fascinating areas of study, physicians of essential knowledge, and women of appropriate care. For example, women’s cardiac care has been adversely affected by failure to recognize that myocardial infarction appears differently in women and men, by the relative failure of physicians to take women’s symptoms seriously, and by the production of cardiac devices so large that they ruptured women’s blood vessels. These are life-and-death issues.