BACKGROUND: An abundance of ovarian hormones is assumed to be a major contributor to the low incidence of ischemic heart disease in premenopausal women. However, the effects of ovarian hormones remain undetermined. OBJECTIVE: To examine whether the variation in ovarian hormone levels throughout a menstrual cycle affects myocardial ischemia in women with variant angina. DESIGN: Prospective, observational study. SETTING: University medical center in Japan. PARTICIPANTS: 10 premenopausal women with variant angina. MEASUREMENTS: Frequency of spontaneous ischemic episodes, flow-mediated dilation of brachial artery, and serum levels of estradiol and progesterone. RESULTS: Frequency of ischemic episodes was highest from the end of the luteal phase to the beginning of the menstrual phase and was lowest in the follicular phase. Flow-mediated vasodilation and estradiol levels were lowest from the end of the luteal phase to the beginning of the menstrual phase and were highest in the follicular phase. CONCLUSIONS: In premenopausal women with variant angina, we documented a cyclic variation in endothelial function and the frequency of myocardial ischemia that was associated with the variation in estrogen levels.