Background: After ileoanal pouch operation, 5% to 40% of patients with ulcerative colitis and 2% to 8% of patients with familial adenomatous polyposis develop pouchitis. Seven percent to 32% of all patients have local septic complications. Pouch ischemia is discussed as a pathophysiologic factor. Tonometry is a minimally invasive method for estimating intramucosal pH (pHi), with a decreased pHi showing intramucosal acidosis characteristic of hypoperfusion. Hypothesis: Decreased perfusion of the iloanal pouch measured by pHi is associated with local septic complications and the development of pouchitis. Design: Prospective cohort study. Setting: Surgical department of a university hospital. Patients and Methods: The pHi in the ileoanal pouch of 98 patients was measured directly after the pouch procedure and correlated to the clinical course. Endoscopic examination of the pouch with biopsy and blinded histologic assessment, including calculation of a histologic pouchitis score, were routinely performed 3 months postoperatively. Main Outcome Measures: Development of pouchitis and local septic complications in correlation to pHi. Results: A decreased pHi was statistically significantly associated with the development of pouchitis and the rate of local septic complications. All 3 patients with anastomotic stenosis had a pHi less than 7.00. The diagnosis of ulcerative colitis just failed in statistical significance as a risk factor for pouchitis. An increased body mass index just failed as a statistically significant risk factor for complications but was a risk factor for the development of acute pouchitis. Conclusion: Pouch hypoperfusion is a risk factor for the development of pouchitis and local septic complications.