Transient neonatal diabetes mellitus (TNDM) is associated with paternal over-expression of an imprinted locus on chromosome 6q24, which contains one differentially methylated region (DMR); maternal demethylation at the DMR accounts for ~20% of cases. Here we report female monozygous triplets, two of whom have TNDM arising from loss of maternal methylation within the TNDM DMR.