Four cases of cysts or tumours developing from embryological remnants in the peri-ano-rectal space are reported. The authors propose for these rare lesions a coherent and simplified classification in two main groups: teratomas on the one hand, developmental cysts on the other hand. The various methods of diagnosis are studied. The precise diagnosis of a congenital cyst can only be made by pathological examination of the specimen after surgical removable. Any rapid local change implied malignant degeneration. Such a risk suggests immediate removal of any known ano-rectal cyst or tumour. The route of approach is usually perineal. Kraske's position gives good access to the retro-ano-rectal space. Possible pelvic spread of these lesions sometimes requires the use of the abdominal approach, either straight away or secondarily when total removal is impossible by the perineal route alone.