We report 3 pairs of monozygotic (MZ) twins, one twin showing typical Wiedemann-Beckwith syndrome (WBS) with minimal or no expression of the condition in the co-twin. These cases are documented, and three previously reported MZ twin pairs are reviewed. Phenotypic concordance for this syndrome in MZ twin pairs has not been reported. Many cases of familial occurrence have been published and different modes of inheritance have been postulated. Based on the twin-twin variability seen in our patients, it seems the most likely mechanism of inheritance is an autosomal dominant mutation with environmental modification of expressivity, or reduced phenotrance.