Monozygous twin births have been associated with brain lesions such as hydranencephaly, multicystic encephalomalacia, and porencephaly. Prenatal circulatory injury has been considered to be the cause. Polymicrogyria is rare but has been reported in autopsied cases. The sibship in this case report, comprising monozygotic male twins and their elder sister from the same non-consanguineous parents, all had global developmental delay. Brain MRI showed polymicrogyria. We suggest that, apart from circulatory compromise, genetic etiology must be implicated as the cause of polymicrogyria.