The etiology of sirenomelia sequence is still obscure. The role of maternal diabetes and a vascular steal phenomenon have been discussed [Gurakan et al. (1996) Turk J Pediatr 38:393-397]. Discordant monozygotic twin sirenomelia has been commonly reported but only rarely in dizygotic twins. The family of the presented twins had a high risk of diabetes mellitus. One of the twins has type 1 sirenomelia and the other had only an imperforate anus. Clin Dysmorphol 9: 227-229 (C) 2000 Lippincott Williams & Wilkins.