Abstract The authors report a case of isolated imperforate anus with perineal fistula in monozygotic twins. Only 4 other well-documented cases of monozygotic twins concordant for isolated anorectal malformation have been reported in the literature. In all these cases the defect occurred below the levator ani muscle. This case confirms previous reports on the incidence of low anorectal anomalies in monozygotic twins where the defect was isolated. A review of the literature relating to this condition in twins and siblings points toward low anomalies and more severe malformations having different genetic backgrounds.