An attempt has been made to correlate motility patterns of the pelvic ileal reservoir with patients' clinical experience. In order to evaluate the motility characteristics of the ileal reservoir, we have related its behaviour to the motility pattern observed in ileum proximal to an ileostomy. Seven patients with an ileostomy and 11 patients with a pelvic ileal reservoir have been investigated. The ileum and ileal reservoir were found to behave similarly during the fasting state and in response to distension and eating; however, a quantitative functional difference was discovered. Distension with 30 ml was enough to produce forceful contractions in the normal ileum, while a mean of 322 ml was necessary to initiate similar activity in the ileal reservoir. The ileal reservoir thus behaves as a capacitance organ with the ability to distend without contraction before a considerable volume has accumulated within it. When it does contract, it initiates a sensation of impending evacuation, but escape of ileal content is opposed by active contraction of the intact external anal sphincter.