Hormonal fluctuations during the menstrual cycle are hypothesized to influence the course of asthma among women. A recent study found that almost 50% of emergency department (ED) visits occur during the perimenstrual phase. Our prospective cohort study in 64 EDs examined the relation between phase of menstrual cycle and visits for acute asthma. A total of 288 women with acute asthma were evaluated with a standardized patient interview and medical record review after excluding subjects who were pregnant, on hormonal therapy, postmenopausal, status post hysterectomy, had incomplete reproductive data, or whose ED visit fell more than 28 d after their last menstrual period. Only 13% reported reproductive factors as a personal asthma trigger. For all subjects, ED asthma visits were classified by menstrual phase: 33% were preovulatory (Days 5 to 11), 26% were periovulatory (Days 12 to 18), 20% were postovulatory (Days 19 to 25), and 21% were perimenstrual (Days 26 to 4), p = 0.008. There was no significant association between phase of menstrual cycle and asthma severity. Our data indicate that ED visits for acute asthma among women are more frequent during the preovulatory phase in contrast to other studies reporting more visits in the perimenstrual phase.