This study was designed to define the different types of strictures, the factors favoring their occurrence, and their treatment after ileal pouch-anal anastomosis. Between January 1981 and June 1996, 1,884 ileal pouch-anal anastomoses were constructed at the Mayo Clinic in Rochester, Minnesota. Data were collected prospectively and included age, gender, type of underlying diseases (chronic ulcerative colitis familial adenomatous polyposis), proctologic antecedents, technique of anastomosis, intraoperative difficulties, and postsurgical complications. Strictures were categorized as nonfibrotic and fibrotic on the basis of the presence or absence of a fibrotic segment at the anal canal anastomosis that was responsible for pouch-outlet obstruction requiring at least one dilation. Strictures occurred in 213 patients (11.2 percent; 11 percent for chronic ulcerative colitis and 12 percent for familial adenomatous polyposis; P = not significant). Strictures were nonfibrotic in 86.4 percent of patients and fibrotic in 13.6 percent. A greater number of strictures were observed after a handsewn anastomosis (12 percent) than after a stapled anastomosis (4 percent; P = 0.03). Intraoperative technical difficulties were associated with 13 percent of all strictures regardless of the type of stricture (fibrotic, 7.5 percent; nonfibrotic, 14 percent; P = 0.4). Postoperative complications such as abscess, fistula, and pouch retraction were found in 13 percent of cases and were primarily associated with fibrotic strictures. Treatment included dilation, which was successful in 95 percent of nonfibrotic strictures but in only 45 percent of fibrotic strictures (P = 0.0001). Surgical treatment was required in 25 strictures (12 percent), including excision of the strictured segment with mucosal advancement flap (5 patients), excision of the pouch with permanent ileostomy (9 patients), or redo pouch (3 patients). With one exception, all excised pouches were associated with other perianastomotic complications, such as abscess, fistula, and pouch retraction. The remaining eight patients had other surgical procedures because of abscess (n = 3), division of an obstructive bridge (n = 2), and débridement and curettage of a fistula (n = 3) with dilation for associated strictures. Strictures were observed in 11.2 percent of the patients in this study. Nonfibrotic strictures responded well to anal dilation, whereas fibrotic strictures were more commonly associated with intraoperative or postoperative complications, often necessitated surgical therapy to salvage pouch function, and were eventually responsible for pouch failure in nine patients.