During a period of eight years, 19 of 83 patients underwent colectomy, mucosal proctectomy and endorectal ileal pull-through procedures with isoperistaltic ileal reservoir for ulcerative colitis or polyposis. These patients had late obstruction of the ileal reservoir develop which lead to distension, stasis, reservoir inflammation and diarrhea. In nine patients, the reservoir obstruction as due to a lengthy rectal muscle canal and a longer distance from the lower end of the reservoir to the anus than was optimal, causing angulation and obstruction. Surgical division of the upper rectal muscle and retraction of the reservoir further into the pelvis combined with resection of the elongated upper end of the ileal reservoir were followed by relief of symptoms in every patient. Five patients had intestinal adhesions develop necessitating resection of a segment of the upper ileal reservoir. Five other patients had obstruction develop from an internal hernia; each required resection of the redundant upper portion of the reservoir. Each of the patients was relieved of diarrhea and gaseous distension after the obstruction was corrected and the ileal reservoir was shortened. The optimal length of ileal reservoir is approximately 10 to 15 centimeters for children and 18 to 22 centimeters for adults. Resection of all except 5 to 7 centimeters of rectal muscle cuff and placement of the lower end of the ileal reservoir within 3 to 5 centimeters of the ileoanal anastomosis appeared to reduce substantially the incidence of reservoir obstruction. Reservoir obstruction has not been observed in the last 37 patients who have undergone operation.