It is now agreed that it is of vital importance to maintain the fascia propria as an intact envelope around the mesorectum to prevent tumor spillage while performing rectal excision for cancer. There are several surgical techniques described to achieve an intact fascial envelope, each arising from differing descriptions of the fascia propria of the rectum. The aim of this study was to describe the detailed surgical anatomy of the fascia propria. Thirteen rectal specimens surgically removed by the technique of extrafascial excision were subjected to gross inspection, dissection, and histologic and electron microscopic examination. The attachments, thickness, and composition of the fascia propria were determined. The fascia propria is a continuous fascial sleeve surrounding the rectum and mesorectum that can be dissected as a complete "sock" off a fresh extrafascial specimen. It is 154 (+/- 1 standard deviation = 61-391) microm thick, is thinner anteriorly than posteriorly (P < 0.05), and is composed predominantly of collagen. It can be identified surgically at the pelvic brim as a shiny membrane and lies inside the hypogastric nerves and the pelvic plexuses. The fascia propria forms a sleeve around the mesorectum, offering a surface against which to dissect, enabling safe removal of the rectum with its intact mesorectum while preserving the autonomic nerves of the pelvis. The term "extrafascial excision" highlights the importance of the fascia propria in this operation.