One hundred and two female asthmatic patients responded to a questionnaire concerning the relationship between their asthma symptoms and the menstrual cycle. Thirty-six subjects stated that their asthma worsened just prior to or at the time of their menses. Daily monitoring of peak expiratory flow rate (PEFR) showed a significant reduction at the time of menstruation in those who worsened compared with those subjects who were unaffected. Cycle length was significantly shorter in the group who worsened but the duration of the menses was not significantly different.