Functional outcome following restorative proctocolectomy depends, in part, on the volume of the pouch. This study investigates the relationship between the length of ileum and volume of saline required to distend the ileum to a set pressure, and the possibility of predicting the volume of an ileoanal pouch using these measurements. Measurements were made on the ileum of 8 mongrel dogs. There was a linear relationship between the length of ileum and volume of saline infused to produce an intraluminal pressure of 20 mmHg (r=0.94-0.99). However, the capacity of ileum varied widely from animal to animal (100 cc of saline filled 15.5 cm to 43 cm of ileum). Twenty-four pouches (15J, 9S) were constructed from segments of ileum of known capacity. The capacity of a J pouch was 1.7±0.11 times greater than the capacity of the original length of bowel and an S pouch had a capacity 2.14±0.25 times greater. These changes in capacity are similar to a theoretical calculation of change in volume of ileum after pouch construction. This technique is a simple, quick method of predicting the volume of an ileo-anal pouch which will be useful in a clinical trial to compare function of pouches of different volumes.