OBJECTIVES: To investigate the reliability of reported stool frequency in the diagnosis of constipation. METHODS: Our criterion for the diagnosis of idiopathic constipation was < or = 3 stools per week for > or = 6 months. Subjects who believed that they met this criterion were invited to participate. Daily stool frequency was recorded over a 4-wk period, and all stools were weighed during the final week. A colon transit study and anorectal manometry were performed at the beginning and at the end of the study. RESULTS: On review of stool diaries from 45 subjects who described chronic constipation and who completed the 4-wk study, only 22 (49%) actually satisfied our criterion for idiopathic constipation. The remaining 23 (51%) subjects, although describing constipation, had, on average, 6 stools/wk. A history of psychiatric illness was 5 times more frequent among those whose bowel symptoms correlated poorly with objective measures. Measures of difficult defecation were similar in the two groups. Mean colon transit time was significantly longer and correlated closely with stool weight only in subjects who truly were constipated (p < 0.05). Anorectal manometry was not helpful in discriminating between those who satisfied our criterion for constipation and those who did not. CONCLUSIONS: Defining constipation on the basis of a patient's reported stool frequency may prove misleading; the diagnosis of idiopathic constipation should be supported by the use of stool diaries and a colon transit study.