Beckwith-Wiedemann syndrome (BWS) presents with visceromegaly, macroglossia, tumor predisposition and other congenital abnormalities, and is usually associated with abnormalities of chromosome 11p15. A number of identical twin pairs, mostly female, have been reported to be discordant for BWS. We show here that the incidence of female monozygotic twins among patients with BWS is dramatically increased over that of the general population. A cluster of imprinted genes within 11p15 is thought to be coordinately regulated via the imprinted expression of KCNQ1OT1, which encodes an untranslated RNA. In skin fibroblasts from five monozygotic twin pairs discordant for BWS, each affected twin had an imprinting defect at KCNQ1OT1 on 11p15, whereas the unaffected twin did not. Five additional monozygotic twin pairs, for whom only blood was available, also displayed an imprinting defect at KCNQ1OT1. It is possible that discordance for BWS in MZ twins is due to unequal splitting of the inner cell mass during twinning, thereby causing differential maintenance of imprinting at KCNQ1OT1. Alternatively, we propose that KCNQ1OT1 is especially vulnerable to a loss of imprinting event, caused by a lack of maintenance DNA methylation at a critical stage of preimplantation development, and that this loss of imprinting predisposes to twinning as well as to discordance for BWS. These data underscore the importance of continued surveillance of children born following assisted reproductive technologies that impact the preimplantation embryo.