Rectal resection with colo-anal anastomosis was performed in 65 patients with carcinoma of the lower rectum. In 20 a pelvic colonic reservoir was constructed while in 45 a direct anastomosis was carried out. There were no postoperative deaths and morbidity was comparable in the two groups. Functional results were determined by clinical examination and manometry. The frequency of bowel movements was inversely related to the maximum tolerated volume (P less than 0.001). During the first year 60 per cent of the patients with a reservoir and 33 per cent of the patients without had one or two stools per day (P less than 0.05). After one year, 86 per cent of the patients with a reservoir and 33 per cent of the patients without had one or two bowel movements per day (P less than 0.01). The maximum tolerated volume was increased by the reservoir (P less than 0.05). The loss of reservoir capacity of the rectum increases frequency of bowel movements in colo-anal anastomosis. The creation of a colonic reservoir improves function by increasing the maximum tolerated volume without any increase in mortality or morbidity.