A pair of monozygotic twins discordant for the Wiedemann-Beckwith syndrome (WBS) associated with congenital central hypothyroidism is described. During pregnancy, the mother was noted to have marked polyhydramnios and severe abdominal distension. At birth, a large diamniotic, monochorionic placenta was noted. According to a study of the minor blood groups and HLA typing, the possibility of monozygosity is 0.985. The mode of inheritance of the syndrome is debatable, and according to observations of discordance in monozygotic twins, single gene control seems unlikely. The thyroid function was noted to be subnormal when the proband was 3 days old. Oral thyroxine has been supplied since infancy. Repair of the umbilical hernia, the bilateral inguinal hernia, and a bilateral orchidopexy were done. A partial glossectomy was later performed to prevent progressive orofacial maldevelopment. The postnatal gigantism persisted as he grew.