Ileo-anal anastomosis (IAA) is a suitable surgical treatment for selected patients with ulcerative colitis or familial adenomatous polyposis. Acute inflammation of the ileal reservoir (pouchitis) is the most frequent late complication and is characterized by endoscopic and histological changes of acute inflammation similar to ulcerative colitis with abdominal and sometimes systemic symptoms. Between May 1984 and April 1993, 160 patients (103 male and 57 female) aged between 7 and 64 years (mean age 32.6) underwent IAA; twenty of these for familial adenomatous polyposis, 138 for ulcerative colitis and 2 for indeterminate colitis. After a mean follow-up of 41.8 months (range 1-108), 138 patients (86.3%) were symptoms free and twenty-two (16 male and 6 female-13.7%) who underwent IAA for ulcerative colitis, had pouchitis. Clinical, biochemical, endoscopic and histologic findings suggest pouchitis as a chronic recurrent disease similar to inflammatory bowel disease. In conclusion, the etiopathology of this new entity is unknown. Bacterial overgrowth is one of the most popular hypothesis, but the onset of the acute inflammation observed by us only in patients with previous ulcerative colitis and in some cases during the period of protective ileostomy suggests a multifactorial pathogenesis.