Anal sphincter dysplasia is a congenital, often familial malformation of the anal canal. In the literature, the anomaly is poorly represented and usually referred to as anteriorly or ventrally displaced anus. The range of symptoms includes chronic constipation, severe straining at defecation, encopresis and chronic paradoxical diarrhea with fecal incontinence. One usually finds dysplasia and absent (type I) or incomplete (type II) fixation of the sphincter complex to the coccyx. Both are demonstrable by computerized tomography (CT) as well as by intraoperative dissection of the sphincter muscles. There ist also shortening of the ectodermal segment of the anal canal which is obviously responsible for the disturbed stool sensation. Posterior butterfly anoplasty combined with fixation of the sphincter complex to the coccyx usually leads to immediate improvement of defecation disorders. A genetic study on 42 patients (age 1-52 years) operated on for anal sphincter dysplasia revealed autosomal dominant inheritance with variable expression and probably incomplete penetrance of the mutated gene.