During the period from 1980 through 1990, our institution constructed 253 ileoanal reservoirs in 253 patients, of whom 25 (9.9%) experienced pouch failure. A poor functional result was the most common cause of pouch failure (seven [28%] of 25 patients). Unsuspected Crohn's disease became manifest in 13 (5%) of the 253 patients, resulting in pouch loss due to perianal sepsis or pouch fistulas in six patients (24% of 25 failures), and resulted in a significantly increased risk of pouch failure compared with that of the non-Crohn's population. Pouchitis occurred in 78 patients (31%) and accounted for four (16%) of 25 failures, but patients with pouchitis were not at higher risk for pouch failure than were patients who did not have pouchitis (failure rates of 6.4% vs 10.4%, respectively; not significant). Significant pelvic sepsis in the absence of Crohn's disease developed in 13 patients, five (38%) of whom lost their pouches. Poor functional results, pelvic sepsis, and unsuspected Crohn's disease were the major causes of pouch failure, while pouchitis was not.