Posterior sagittal anorectoplasty provides an optimal access to reconstruct the muscle complex in anorectal malformations. It gives much better results than the abdominoperineal pullthrough procedures performed before 1984. However, severe chronic constipation occurs postoperatively in about 10 % of the patients, which can only be treated by washouts. Clinical investigations of 578 patients treated from 1962 to 1984 and from 1985 to 1997 are presented here and both groups are compared to each other. In addition, a new continence score with special regard to chronic constipation and overflow incontinence was used to follow up 133 patients of the second group. The score distinguishes between children above and below the age of 3 years. To study the underlying reasons of severe chronic constipation in children with anorectal malformations, macro- and microanatomical investigations on 4 normal newborns, 3 neonatal piglets with imperforate anus and 25 rectal biopsies from the caecum were performed. The following reasons have been found to be probably responsible for postoperative constipation and overflow incontinence: 1. Malformations of the smooth and striated muscle fibres or connective tissue of the caecum; 2. Malformations of the intramural nerve plexus such as aganglionosis, hypoganglionosis or IND; 3. Malformations and/or iatrogenic lesions of the extramural nerve supply which runs anterior to the rectum and in front of the fascia of Denonvilliers, which can hardly be identified in neonates with imperforate anus. Therefore iatrogenic bladder injuries may occur after PSARP after extended mobilisation of the caecum. The macro- and microanatomical situation in the piglet with imperforate anus is totally different from the human newborn.