Teixeira RJPB, et al. – A cross–sectional study was conducted to assess the agreement of molar incisor hypomineralization (MIH) between monozygotic and dizygotic twin pairs and the relationship with environmental factors. The greater concordance in the diagnosis of MIH among monozygotic twins showed a genetic influence. Although environmental factors, such as family income and hemorrhage during pregnancy, were also related to the occurrence of MIH.