In recent clinical trials of medical therapy for heart failure, only approximately 20% of patients enrolled were women. The reasons for the low enrollment of women have not been clear. Although the incidence of heart failure is higher in men than in women, the prevalence is equal. When men and women with heart failure and a low left ventricular ejection fraction are compared, the women are more symptomatic and have a similarly poor outcome. Because mortality is worse in men than in women in large populations of patients with heart failure, there may be important pathophysiologic differences. Substantial data suggest that women may have diastolic dysfunction more often than men. This difference would explain differences in mortality and the difficulty in enrolling women in studies of medical therapy for heart failure with underlying systolic dysfunction.