Urinary tract dysfunction influenced by the menstrual cycle has been described. This case demonstrates mensturation-influenced fecal incontinence. The patient had difficulty with stool control and rectal hygiene only during the week prior to her period. These symptoms had been present since she had discontinued oral birth control. Progestin levels and anal manometry were obtained during the secretory and luteal phases of her cycle and demonstrated a difference in resting anal tone between phases. Progestin smooth muscle relaxation is presumed to have uncovered an already weakened anal continence mechanism. With reinstitution of oral contraceptive pills the patient's cyclic fecal incontinence resolved.