Many women report that bowel symptoms are associated with menstruation, but neither the prevalence of these complaints nor their physiological basis is known. This study aimed to estimate prevalence, to determine whether patients with irritable bowel syndrome are more likely to make such complaints, and to determine whether bowel complaints during menstruation are attributable to psychological traits such as increased somatization. To estimate prevalence, 369 clients of Planned Parenthood of Maryland were asked whether gas, diarrhea, or constipation occurred during menstruation. These subjects were compared with women referred to a gastroenterology clinic and found to have irritable bowel syndrome or functional bowel disorder (abdominal pain plus altered bowel habits but not satisfying restrictive criteria for irritable bowel syndrome). Thirty-four percent of 233 Planned Parenthood clients who denied symptoms of irritable bowel syndrome or functional bowel disorder reported that menstruation was associated with one or more bowel symptoms. Gastroenterology clinic patients with irritable bowel syndrome were significantly more likely to experience exacerbations of each of these bowel symptoms, but especially increased bowel gas. Self-reports of bowel symptoms during menstruation were not associated with psychological traits or with menses-related changes in affect.