Case report of an extremely rare retrorectal duplication, containing gastric mucosa folds with a fistula to the rectum in a 10-year-old boy with myelodysplasia syndrome, anal stenosis and a sacral defect. Persistent tar-like stools prompted detailed investigations, followed by extirpation of the doubling. The combination of anorectal malformations, presacral masses and sacrococcygeal defect may point to a Currarino triad, an autosomal dominant hereditary syndrome which can be simply detected or identified by x-ray examination of the os sacrum.