The aim of the investigation was to determine the most functionally grounded level of the formation of colonic reservoir from the margin of the anal canal while performing a low anterior resection of the rectum. Sphincter-preserving operations were performed in 113 patient with the formation of colonic reservoir and direct stepler colorectal anastomosis. Questionnaires and apparatus "Colodynamic-3" were used to follow-up 28 patients with the reservoir (main group) and 19 patients with a direct anastomosis on whom the operations had been made more than a year ago. The rate of the development of complications was almost the same in the both groups--28.5% and 26% correspondingly. The reservoir-accumulating function became less in the group of "direct" anastomoses. The best parameters of the reservoir-accumulating and evacuatory ability of the colonic reservoir were found to be in patients with the level of anastomoses 4-6 cm from the anal canal margin. Creation of the reservoir was more difficult in patients with the carneus mesentery and a small width of the sigmoid colon.