The cause of an anal fissure, and particularly the reason why the posterior midline is so frequently affected, is not com- pletely understood. Classically, acute anal fissures arise from the trauma caused by the strained evacuation of a hard stool or, less commonly, from the repeated passage of diarrhoea. The location in the posterior midline perhaps relates to the exagger- ated shearing forces acting at that site at defaecation, combined with a less elastic anoderm endowed with an increased density of longitudinal muscle extensions in that region of the anal circumference. Anterior anal fissure is much more common in women and may arise following vaginal delivery. Perpetuation and chronicity may result from repeated trauma, anal hyperto- nicity and vascular insufficiency, either secondary to increased sphincter tone or because the posterior commisure is less well perfused than the remainder of the anal circumference