BACKGROUND/AIMS: The female sex hormones are thought to affect gastrointestinal function. However, the relationship between female sex hormone and gastrointestinal function has not been identified. The aim of this study was to evaluate the relationship between constipation and hormonal changes in young women and to find the difference of colon transit time (CTT) across the menstrual cycle. METHODS: Two hundred and five students completed questionnaires. CTT as well as serum estradiol and progesterone levels were measured in 15 regularly menstruating volunteers. RESULTS: One hundred and six experienced changes in bowel habit across menstrual cycle. The mean estradiol concentration in the follicular phase was not significantly different from that in the luteal phase (118.0 +/- 28.8 pg/mL vs. 76.9 +/- 10.5 pg/mL, p > 0.05). The mean progesterone concentration in the luteal phase was significantly higher than that in the follicular phase (3.0 +/- 0.9 ng/mL vs. 1.2 +/- 0.4 ng/mL, p < 0.05). The mean CTT was not significantly delayed in the luteal phase than the time in the follicular phase (45 +/- 20 vs. 35 +/- 22 hours, p > 0.05). However, CTT was delayed at the progesterone peak in 11 of 15 (73.3%) women. CONCLUSIONS: Our study suggested a possible role of female sex hormones as a cause of constipation in young women.