Anorectal malformation (ARM) can be divided in high, intermediate, and low forms according to the level of termination of the rectum in relation to the pubococcygeal and ischiatic lines. Patients with Down's syndrome have a high incidence of gastrointestinal anomalies, such as tracheoesophageal fistula, duodenal obstruction, annular pancreas, Hirschsprung's disease, and ARM. In these children, ARM is generally low with or without a fistula. The mode of inheritance of ARM and its genetic relation with Down's syndrome is not known, even if the association (ARM-Down's syndrome) seems not to be coincidental. We describe here a very rare case of monozygotic twins born with the association of ARM and Down's syndrome.