BACKGROUND: No agreement has been found in the literature concerning the safest point of ligation of the inferior mesenteric artery (ima) in order to avoid nerve damage during the surgery of rectal cancer. STUDY DESIGN: The distance between the origin of the ima and the left paraortic trunk was measured, as was the distance between the left paraortic trunk and the origin of the left colic artery (lca). The measurements were carried out on 20 cadavers and during 22 operations for rectal cancer. RESULTS: The left paraortic trunk always runs posterior to the ima: its distance from the origin of the ima is on average 1.2 cm; the distance of the left paraortic trunk from the origin of the lca is on average 0.4 cm. The point at which the ima and the left paraortic trunk cross varies greatly, but it is never near the origin of the ima. CONCLUSIONS: From an anatomical point of view the safest point of ligation of the ima is at its origin. At this point, the left paraortic trunk never runs; so there isn't any risk to damage the nerve involving it during the ligation of the artery.