RATIONALE: There is little knowledge of variations in respiratory symptoms during the menstrual cycle in a general population, and no investigation of potential modifying factors. OBJECTIVE: To investigate menstrual cycle variation in respiratory symptoms in a large general population, using chronobiology methodology, and stratifying by BMI, smoking and asthma status. METHODS: 3926 women with regular cycles

/=23kg/m2 and smokers, or just prior to ovulation and menses onset for low symptomatic subgroups. CONCLUSIONS: Respiratory symptoms varied significantly during the menstrual cycle and were most frequent from the mid-luteal to mid-follicular stages, often with a dip near the time of ovulation. The patterns varied by BMI, smoking and asthma status. These relations link respiratory symptoms with hormonal changes through the menstrual cycle and imply a potential for individualized chronotherapy for respiratory diseases