Background and Objectives: To assess whether fecal pH might be an indicator of pouchitis during the postoperative period in hereditary colorectal cancer (CRC) patients who have undergone ileal pouch anal anastomosis (IPAA). Methods: Five consecutive daily pH values of stool samples from 31 familial adenomatous polyposis (FAP) patients and 32 metachronous Lynch syndrome patients who underwent IPAA procedures were reviewed. Patients with pouchitis (pouchitis group, n = 22) were compared with patients without pouchitis (nonpouchitis group, n = 41). A receiver operating characteristic (ROC) analysis was performed to determine the indicative potential of fecal pH for pouchitis. A Mantel-Cox test was also performed to evaluate the survival status of patients with or without pouchitis. Results: Pouchitis was noted in 22 (34.9%) of 63 patients after IPAA. The significance of each daily average fecal pH value and the 5-day overall average fecal pH value was compared between the two groups (P < .01). A cutoff fecal pH value of 7.46 was determined by the ROC analysis for assessing the risk of pouchitis. No significant difference in 5-year overall survival was observed between the two groups. Conclusion: A lower fecal pH value in patients with hereditary CRC after IPAA might be a new indicator of pouchitis. K E Y W O R D S hereditary colorectal cancer, IPAA, pH value, pouchitis